JP2010019138A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an air-fuel ratio control device for an internal combustion engine that can suppress the deterioration of exhaust gas emissions during responsiveness diagnosis operation, with respect to the internal combustion engine performing the responsiveness diagnosis operation of an air-fuel ratio sensor disposed at an upstream side of a catalyst. <P>SOLUTION: A start catalyst 44, an under-floor catalyst 45, an A/F sensor 46 at the upstream side of the start catalyst 44, and an O<SB>2</SB>sensor 47 at the downstream side of the start catalyst 44 are provided in an exhaust system. If O<SB>2</SB>atmospheres in the respective catalysts 44, 45 are not in favorable states for exhaust gas purification, when the execution condition of A/F active control for diagnosing the responsiveness of the A/F sensor 46 is established, the A/F active control is prohibited so that the deterioration of exhaust gas emissions is prevented. Furthermore, if the inside of the catalyst is lean, a target air-fuel ratio is shifted to a rich side. If the inside of the catalyst is rich, the target air-fuel ratio is shifted to a lean side so that the A/F active control is started earlier. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention relates to an air-fuel ratio control device for an internal combustion engine represented by an automobile engine. In particular, the present invention relates to a measure for improving exhaust emission in an internal combustion engine capable of performing a responsiveness diagnosis operation on an air-fuel ratio sensor disposed in an exhaust system.

  Conventionally, for example, as disclosed in Patent Document 1 and Patent Document 2 below, in an exhaust system of an automobile engine, an air-fuel ratio sensor (hereinafter referred to as an A / F sensor) is disposed upstream of a catalyst (for example, a three-way catalyst). It is known that the air-fuel ratio feedback control is performed based on the output of the A / F sensor. That is, when the output of the A / F sensor is on the lean side with respect to the target air-fuel ratio (for example, the theoretical air-fuel ratio 14.6), the fuel injection amount is increased and corrected, while the output of the A / F sensor is on the rich side. If so, the fuel injection amount reduction correction is performed. As a result, the actual air-fuel ratio is brought close to the target air-fuel ratio to improve the exhaust emission.

In Patent Document 3 below, an oxygen concentration sensor (hereinafter sometimes simply referred to as an oxygen sensor) is provided on the downstream side of the three-way catalyst, and the three-way catalyst is disposed further downstream of the oxygen sensor. And further improvement of exhaust emission is disclosed. Generally, the upstream catalyst is referred to as a start catalytic converter (hereinafter referred to as start catalyst), and the downstream catalyst is referred to as an under floor catalytic converter (hereinafter referred to as under floor catalyst). These three-way catalysts have an O ₂ storage function (oxygen storage function) for storing (occluding) oxygen. Therefore, when the air-fuel ratio of the inflowing exhaust gas is rich, unburned components such as HC and CO are oxidized by the stored oxygen, while when the air-fuel ratio of the inflowing exhaust gas is lean Nitrogen oxide (NOx) is reduced, and oxygen taken from the NOx is stored inside the catalyst. As a result, the three-way catalyst can effectively purify the unburned components and nitrogen oxides even when the actual air-fuel ratio of the engine deviates from the stoichiometric air-fuel ratio.

  As described above, the air-fuel ratio control when the A / F sensor is provided on the upstream side of the catalyst (start catalyst) and the oxygen sensor is provided on the downstream side is based on the output signal of the A / F sensor. In addition to the main feedback control in which the fuel injection amount is controlled so that the air-fuel ratio of the exhaust gas flowing into the engine becomes the target air-fuel ratio (for example, the theoretical air-fuel ratio), the output signal of the A / F sensor is based on the output signal of the oxygen sensor. Sub-feedback control for correcting the above is performed. Note that the correction amount in the sub-feedback control (hereinafter sometimes referred to as a learning value) is calculated as a value corresponding to a change with time of the engine or the like, and is updated each time the sub-feedback learning operation is performed. By executing such feedback control, for example, the fuel injection amount from the injector is adjusted so that the actual air-fuel ratio becomes the stoichiometric air-fuel ratio, and emission of exhaust gas is improved.

  Incidentally, in order to ensure the reliability of the air-fuel ratio feedback control as described above, it is necessary that the responsiveness of the A / F sensor is sufficiently high. Therefore, the responsiveness diagnosis operation of the A / F sensor is performed.

  Specifically, as this responsiveness diagnosis operation, the target air-fuel ratio is changed to the rich side (for example, air-fuel ratio 14.1) with respect to the target air-fuel ratio (for example, theoretical air-fuel ratio = 14.6) before the start of the diagnosis operation. The operation to change to the lean side (for example, the air-fuel ratio 15.1) is alternately switched in a short time (for example, 0.6 sec), and the output change of the A / F sensor corresponding to the change (change of the output voltage) The responsiveness of the A / F sensor is diagnosed based on the above (generally called A / F active control). In other words, when the A / F sensor has high responsiveness, a large amount of change in sensor output per unit time can be obtained, whereas the responsiveness of the A / F sensor is insufficient. The amount of change in sensor output per unit time is small. By determining these, the responsiveness of the A / F sensor is diagnosed.

FIG. 7 shows a change in the output voltage of the A / F sensor when the A / F active control is executed. A one-dot chain line in the figure is the target air-fuel ratio, and is switched between the rich side and the lean side every predetermined period. In addition, the solid line in the figure is a waveform in the case where high responsiveness is obtained in the A / F sensor, and a large amount of change in sensor output per unit time is obtained. On the other hand, the broken line in the figure is a waveform when the response of the A / F sensor is not sufficiently obtained, and the change amount of the sensor output per unit time is small.
JP-A-2005-121003 JP 2004-204772 A JP 2003-97334 A

  As described above, the responsiveness diagnosis operation (A / F active control) of the A / F sensor is performed. Until now, the above responsiveness is satisfied when a predetermined diagnosis operation execution condition is satisfied regardless of the state of the catalyst. A diagnostic operation was performed. The conditions for executing the diagnostic operation include, for example, that the warm-up operation of the engine has been completed, the fluctuation amount of the engine speed and the intake air amount is small, and the A / F sensor has reached a predetermined activation temperature. Etc.

  The inventor of the present invention has found that when the responsiveness diagnosis operation of the A / F sensor is performed in accordance with such a diagnostic operation execution condition, exhaust emission may be deteriorated.

  That is, when the target air-fuel ratio is switched to the lean side when the responsiveness diagnosis operation of the A / F sensor is performed when the oxygen concentration atmosphere inside the catalyst is lean, NOx contained in the exhaust gas is reduced in the catalyst. It has been found that this NOx may be released into the atmosphere.

  Conversely, when the responsiveness diagnosis operation of the A / F sensor is performed with the oxygen concentration atmosphere inside the catalyst being rich, when the target air-fuel ratio is switched to the rich side, the HC and CO contained in the exhaust are It has also been found that there is a possibility that HC and CO may be released into the atmosphere because the catalyst is less likely to be oxidized.

  The present invention has been made in view of such a point, and an object of the present invention is to provide this responsiveness in an internal combustion engine capable of performing a responsiveness diagnosis operation on an air-fuel ratio sensor disposed upstream of the catalyst. An object of the present invention is to provide an air-fuel ratio control apparatus for an internal combustion engine that can suppress deterioration of exhaust emission during a diagnostic operation.

-Principle of solving the problem-
The solution principle of the present invention taken to achieve the above object is to switch between the case where the responsiveness diagnosis operation of the air-fuel ratio sensor is prohibited and the case where it is permitted depending on the oxygen concentration atmosphere inside the catalyst. Yes. In other words, if the oxygen concentration atmosphere inside the catalyst is not suitable for exhaust gas purification (if the oxygen amount inside the catalyst is too high, the reduction reaction cannot be sufficiently performed, or if the oxygen amount is too low, the oxidation reaction In such a situation, the responsiveness diagnosing operation is prohibited, because there is a concern that exhaust emission may deteriorate due to the execution of the responsiveness diagnosing operation. In addition, when the responsiveness diagnostic operation is prohibited in this way, the responsiveness diagnostic operation is executed early by correcting the air-fuel ratio so that the oxygen concentration atmosphere inside the catalyst is suitable for exhaust gas purification. I am trying to make it possible.

-Solution-
Specifically, the present invention is provided with an air-fuel ratio sensor that outputs a signal corresponding to the oxygen concentration of the exhaust gas upstream of the catalyst upstream of the catalyst that purifies the exhaust gas of the internal combustion engine. It is assumed that the air-fuel ratio control apparatus for an internal combustion engine is configured to forcibly switch the air-fuel ratio of the internal combustion engine alternately between the rich side and the lean side during the responsiveness diagnosis operation. With respect to this air-fuel ratio control device, an oxygen concentration atmosphere determination means for determining whether or not the oxygen concentration atmosphere inside the catalyst is a predetermined oxygen concentration atmosphere suitable for exhaust gas purification, and the oxygen concentration atmosphere determination means And a diagnostic operation prohibiting means for prohibiting the responsiveness diagnostic operation of the air-fuel ratio sensor when it is determined that the oxygen concentration atmosphere inside the catalyst is not a predetermined oxygen concentration atmosphere suitable for exhaust gas purification. Yes.

  The “predetermined oxygen concentration atmosphere suitable for exhaust gas purification” here refers to a state in which NOx purification by a reduction reaction inside the catalyst and HC and CO purification by an oxidation reaction can be performed satisfactorily. In this state, about half (for example, 40% to 60%) of oxygen is occluded with respect to the oxygen storage limit inside the catalyst. This value is not limited to this.

  When the execution condition of the responsiveness diagnosis operation of the air-fuel ratio sensor is satisfied by this specific matter, the oxygen concentration atmosphere determination means is such that the oxygen concentration atmosphere inside the catalyst is in a predetermined oxygen concentration atmosphere suitable for exhaust gas purification. It is determined whether or not. When the oxygen concentration atmosphere is suitable for purifying exhaust gas, the responsiveness diagnosis operation of the air-fuel ratio sensor is started as it is. That is, the air-fuel ratio of the internal combustion engine is forcibly switched alternately between the rich side and the lean side, and the responsiveness of the air-fuel ratio sensor is diagnosed based on the change in the output of the air-fuel ratio sensor accordingly. When the change amount of the sensor output per unit time is large, it is diagnosed that the air-fuel ratio sensor has high responsiveness. When the change amount of the sensor output per unit time is small, Diagnose that the air-fuel ratio sensor has insufficient response.

  On the other hand, even if the execution condition of the response diagnostic operation of the air-fuel ratio sensor is satisfied, if the oxygen concentration atmosphere inside the catalyst is not in a predetermined oxygen concentration atmosphere suitable for exhaust gas purification, The responsiveness diagnostic operation of the air-fuel ratio sensor is prohibited by the diagnostic operation prohibiting means. As a result, when the above-described responsiveness diagnosis operation is performed with the oxygen concentration atmosphere inside the catalyst being lean, or when the oxygen concentration atmosphere inside the catalyst is rich and the above responsiveness When a diagnostic operation is performed, a situation in which HC and CO are released into the atmosphere can be avoided, and deterioration of exhaust emission can be prevented.

  As described above, when the responsiveness diagnosing operation of the air-fuel ratio sensor is prohibited, means for shifting to a state where the responsiveness diagnosing operation can be executed at an early stage can be mentioned. That is, when the oxygen concentration atmosphere determination means determines that the oxygen concentration atmosphere inside the catalyst is not a predetermined oxygen concentration atmosphere suitable for exhaust gas purification, this oxygen concentration atmosphere is determined as a predetermined value suitable for exhaust gas purification. An air-fuel ratio correcting means for performing an air-fuel ratio correcting operation is provided so as to approach the oxygen concentration atmosphere. Then, when the air-fuel ratio correction operation by the air-fuel ratio correction means causes the oxygen concentration atmosphere inside the catalyst to become a predetermined oxygen concentration atmosphere suitable for purification of exhaust gas, the air-fuel ratio sensor by the diagnostic operation prohibiting means In this configuration, the prohibition state of the responsiveness diagnosis operation is released, and the execution of the responsiveness diagnosis operation of the air-fuel ratio sensor is permitted.

  For example, when the responsiveness diagnosis operation is prohibited because the atmosphere of oxygen concentration inside the catalyst is lean, the air-fuel ratio correction operation (change of the target air-fuel ratio) is performed so as to shift the air-fuel ratio to the rich side. Done. On the other hand, when the responsiveness diagnosis operation is prohibited due to the rich oxygen concentration atmosphere inside the catalyst, the air-fuel ratio correction operation (change of the target air-fuel ratio) is performed so as to shift the air-fuel ratio to the lean side. Done. As a result, the oxygen concentration atmosphere inside the catalyst is quickly improved, and the oxygen concentration atmosphere inside the catalyst reaches a predetermined oxygen concentration atmosphere suitable for exhaust gas purification within a relatively short time. To do. Thereby, the prohibition state of the responsiveness diagnostic operation is released, and the execution of the responsiveness diagnostic operation of the air-fuel ratio sensor is permitted. In this way, by actively performing the control operation to improve the oxygen concentration atmosphere inside the catalyst, the responsiveness diagnosis operation is performed within a short period of time after the execution condition of the responsiveness diagnosis operation of the air-fuel ratio sensor is established. It becomes possible to execute.

  Specific examples of the configuration of the catalyst and the air-fuel ratio correcting operation by the air-fuel ratio correcting means include the following. The catalyst is composed of an upstream catalyst and a downstream catalyst connected in series. The air-fuel ratio correction means performs an air-fuel ratio correction operation so that the oxygen concentration atmosphere in the downstream catalyst approaches a predetermined oxygen concentration atmosphere suitable for exhaust gas purification, and the oxygen in the downstream catalyst is corrected. After the concentration atmosphere has become a predetermined oxygen concentration atmosphere suitable for exhaust gas purification, the air-fuel ratio correction is performed so that the oxygen concentration atmosphere in the upstream catalyst approaches the predetermined oxygen concentration atmosphere suitable for exhaust gas purification. It is set as the structure which performs operation | movement.

  When the upstream side catalyst and the downstream side catalyst are provided in the exhaust system in this way, it is possible to further improve the exhaust emission. In this configuration, since the upstream catalyst is close to the combustion chamber, the responsiveness of the change in the oxygen concentration atmosphere when the air-fuel ratio correcting operation is performed by the air-fuel ratio correcting means is high. On the other hand, since the downstream catalyst is far from the combustion chamber, the responsiveness of the change in the oxygen concentration atmosphere when the air-fuel ratio correcting operation is performed by the air-fuel ratio correcting means is lower than that of the upstream catalyst. For this reason, after performing an air-fuel ratio correction operation so that the oxygen concentration atmosphere in the upstream catalyst becomes a predetermined oxygen concentration atmosphere suitable for purification of exhaust gas, the oxygen concentration atmosphere in the downstream catalyst If the air-fuel ratio correction operation is performed so as to obtain a predetermined oxygen concentration atmosphere suitable for purification, there is a possibility that the oxygen concentration atmosphere inside the upstream catalyst will deviate from an appropriate state during the control of the downstream catalyst. In addition, there is a possibility that the exhaust emission may be deteriorated due to the deviation at the time of the responsiveness diagnosis operation of the air-fuel ratio sensor. For this reason, in the present solution, the air-fuel ratio correction operation is performed first to bring the oxygen concentration atmosphere inside the downstream side catalyst closer to a predetermined oxygen concentration atmosphere suitable for purification of exhaust gas. As a result, both the upstream catalyst and the downstream catalyst start the responsiveness diagnosis operation in a state where the oxygen concentration atmosphere becomes a predetermined oxygen concentration atmosphere suitable for exhaust gas purification, and avoid deterioration of exhaust emission. Can do.

  Specific examples of the operation for determining whether the oxygen concentration atmosphere inside the catalyst is a predetermined oxygen concentration atmosphere suitable for exhaust gas purification include the following. That is, the catalyst is composed of an upstream catalyst and a downstream catalyst connected in series. An oxygen sensor that outputs a rich signal or a lean signal corresponding to the oxygen concentration of the exhaust gas is disposed between the upstream catalyst and the downstream catalyst. A signal corresponding to the oxygen concentration of the exhaust gas on the downstream side of the upstream catalyst outputted from the oxygen sensor, and the estimated oxygen concentration value on the downstream side of the downstream catalyst estimated based on this signal are both predetermined. If it is within the range, the air-fuel ratio sensor responsiveness diagnosis operation is executed assuming that the oxygen concentration atmosphere inside the upstream catalyst and the downstream catalyst are both in a predetermined oxygen concentration atmosphere suitable for exhaust gas purification. I have to.

  In the present invention, when the oxygen concentration atmosphere inside the catalyst is not suitable for exhaust gas purification, there is a concern that exhaust emission will deteriorate with the execution of the responsiveness diagnosis operation of the air-fuel ratio sensor. Sex diagnostic operation is prohibited. For this reason, it is possible to avoid the situation where NOx, HC and CO are released into the atmosphere when the responsiveness diagnostic operation is performed, and prevent deterioration of exhaust emissions associated with the execution of the responsiveness diagnostic operation. can do.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the present embodiment, a case where the air-fuel ratio control apparatus according to the present invention is applied to a four-cylinder gasoline engine (internal combustion engine) for an automobile will be described.

-Engine configuration description-
First, a schematic configuration of an engine (internal combustion engine) 1 according to the present embodiment will be described with reference to FIG. As shown in FIG. 1, the engine 1 according to the present embodiment includes a cylinder block 11 having cylinder bores 11 a for four cylinders (only one cylinder is shown in FIG. 1), and a cylinder head 12. Each cylinder bore 11a is provided with a piston 13 provided so as to be movable up and down, and this piston 13 is connected to a crankshaft 15 which is an output shaft of the engine 1 via a connecting rod (connecting rod) 14. A combustion chamber 16 is defined by a space surrounded by the piston 13 and the cylinder head 12 inside the cylinder bore 11a.

  A spark plug (a spark plug) 2 is attached to the cylinder head 12 corresponding to each combustion chamber 16. The cylinder head 12 is provided with an intake port 12a and an exhaust port 12b communicating with each combustion chamber 16, and an intake passage 3 and an exhaust passage 4 are connected to the intake port 12a and the exhaust port 12b, respectively. . An intake valve 31 and an exhaust valve 41 are provided at each open end leading to the combustion chamber 16 in the intake port 12a and the exhaust port 12b. The intake valve 31 and the exhaust valve 41 are opened and closed by an intake camshaft 32 and an exhaust camshaft 42 that are respectively rotated by the power of the crankshaft 15. The power of the crankshaft 15 is transmitted to the intake camshaft 32 and the exhaust camshaft 42 via the timing belt 15a and the timing pulleys 33 and 43.

  Further, in the vicinity of the intake port 12a, a fuel injection valve (injector) 5 is provided corresponding to each cylinder. Each injector 5 is supplied with fuel of a predetermined pressure via a fuel supply system.

  On the other hand, a surge tank 34 is provided in the intake passage 3, and a throttle valve 36 whose opening degree is adjusted by driving a throttle motor 36a is provided on the upstream side of the surge tank 34. The amount of intake air introduced into the intake passage 3 is adjusted according to the opening of the throttle valve 36. Furthermore, an air cleaner 37 for purifying the intake air is provided on the upstream side of the throttle valve 36.

  When the operation of the engine 1 is started, the intake air is introduced into the intake passage 3 and fuel is injected from the injector 5, whereby the intake air and the fuel are mixed to form an air-fuel mixture. In the intake stroke of the engine 1, the intake port 12a is opened by the intake valve 31, whereby the air-fuel mixture is taken into the combustion chamber 16 through the intake port 12a. The air-fuel mixture taken into the combustion chamber 16 is compressed in the compression stroke, and then ignited by the spark plug 2. The air-fuel mixture explodes and burns, and a driving force is applied to the crankshaft 15 (expansion stroke). . The exhaust gas after combustion is discharged to the exhaust passage 4 (exhaust stroke) by opening the exhaust port 12b by the exhaust valve 41, further purified through the catalysts 44 and 45, and then released to the outside. The spark plug 2 executes an ignition operation for the air-fuel mixture in accordance with the application timing of the high voltage output from the igniter 21.

Two three-way catalysts 44 and 45 are disposed in the exhaust passage 4 of the engine 1. These three-way catalysts 44 and 45 have an O ₂ storage function (oxygen storage function) for storing (storing) oxygen, and it is assumed that the air-fuel ratio has shifted from the stoichiometric air-fuel ratio to a certain extent by this oxygen storage function. In addition, HC, CO and NOx can be purified. That is, when the air-fuel ratio of the engine 1 becomes lean and oxygen and NOx in the exhaust gas flowing into the three-way catalysts 44 and 45 increase, the three-way catalysts 44 and 45 occlude part of the oxygen, so that the NOx. Promote reduction and purification. On the other hand, when the air-fuel ratio of the engine 1 becomes rich and the exhaust gas flowing into the three-way catalysts 44 and 45 contains a large amount of HC and CO, the three-way catalysts 44 and 45 store oxygen molecules stored therein. , Giving oxygen molecules to these HC and CO to promote oxidation and purification.

  A three-way catalyst located upstream in the exhaust system is a start catalyst (upstream catalyst) 44. Since the start catalyst 44 is provided on the upstream side of the exhaust passage 4 (on the side close to the combustion chamber 16), the start catalyst 44 has a feature that it rises to the activation temperature within a short time after the engine 1 is started. The three-way catalyst located downstream in the exhaust system is an underfloor catalyst (downstream catalyst) 45. The underfloor catalyst 45 is for purifying HC, CO and NOx that could not be purified by the start catalyst 44, and is disposed below the floor panel constituting the vehicle body.

  An A / F sensor (air-fuel ratio sensor) 46 is disposed upstream of the start catalyst 44 in the exhaust passage 4. For example, a limiting current type oxygen concentration sensor is applied to the A / F sensor 46, and an output voltage corresponding to the air-fuel ratio is generated over a wide air-fuel ratio region.

An O ₂ sensor (oxygen sensor) 47 is disposed on the exhaust passage 4 downstream of the start catalyst 44 and upstream of the underfloor catalyst 45. For this O ₂ sensor 47, for example, an electromotive force type (concentration cell type) oxygen concentration sensor is applied, and the output value changes stepwise in the vicinity of the theoretical air-fuel ratio.

The signals generated by the A / F sensor 46 and the O ₂ sensor 47 are A / D converted and then input to the engine ECU 6.

  The start catalyst 44 is provided with a catalyst temperature sensor 48 for detecting the catalyst temperature.

-Description of control block-
The operating state of the engine 1 is controlled by an engine ECU (Electronic Control Unit) 6. As shown in FIG. 2, the engine ECU 6 includes a CPU (Central Processing Unit) 61, a ROM (Read Only Memory) 62, a RAM (Random Access Memory) 63, a backup RAM 64, and the like.

  The ROM 62 stores various control programs, maps that are referred to when the various control programs are executed, and the like. The CPU 61 executes arithmetic processing based on various control programs and maps stored in the ROM 62.

  The RAM 63 is a memory that temporarily stores calculation results in the CPU 61, data input from each sensor, and the like. The backup RAM 64 is a non-volatile memory that stores data to be saved when the engine 1 is stopped. is there. The ROM 62, CPU 61, RAM 63, and backup RAM 64 are connected to each other via a bus 67, and are also connected to an external input circuit 65 and an external output circuit 66.

The external input circuit 65 includes an accelerator opening sensor 70, a water temperature sensor 71, an air flow meter 72, an intake air temperature sensor 73, an A / F sensor 46, an O ₂ sensor 47, a throttle position sensor 75, a crank angle sensor 76, and a cam angle sensor. 77, a knock sensor 78, and an intake pressure sensor 79 are connected to each other. On the other hand, the external output circuit 66 is connected to the injector 5, the igniter 21, the throttle motor 36a for driving the throttle valve 36, and the like.

  The accelerator opening sensor 70 detects the opening (operation amount) of the accelerator pedal 35 operated by the driver, and transmits the opening signal to the engine ECU 6.

  The water temperature sensor 71 detects the temperature of the cooling water flowing in the water jacket 11b formed in the cylinder block 11, and transmits the cooling water temperature signal to the engine ECU 6.

  The air flow meter 72 detects the intake air amount and transmits the intake air amount signal to the engine ECU 6.

  The intake air temperature sensor 73 is provided integrally with the air flow meter 72, detects the intake air temperature, and transmits the intake air temperature signal to the engine ECU 6.

  The A / F sensor 46 is a limiting current type oxygen concentration sensor as described above, and transmits an output voltage corresponding to the air-fuel ratio to the engine ECU 6.

The O ₂ sensor 74 is a concentration cell type oxygen concentration sensor as described above, and determines whether or not the air-fuel ratio in the exhaust is at the stoichiometric air-fuel ratio, and transmits a determination signal to the engine ECU 6.

  The throttle position sensor 75 detects the opening of the throttle valve 36 and transmits a throttle opening detection signal to the engine ECU 6.

  The crank angle sensor 76 is disposed in the vicinity of the crankshaft 15 and detects the rotation angle (crank angle CA) and the rotation speed (engine speed Ne) of the crankshaft 15. Specifically, the crank angle sensor 76 outputs a pulse signal every predetermined crank angle (for example, 30 °). As an example of a crank angle detection method by the crank angle sensor 76, external teeth are formed at intervals of 30 ° on the outer peripheral surface of a rotor (NE rotor) integrally rotated with the crankshaft 15 and face the external teeth. The crank angle sensor 76 formed of an electromagnetic pickup is disposed. When the external teeth pass near the crank angle sensor 76 as the crankshaft 15 rotates, the crank angle sensor 76 generates an output pulse. In addition, as this NE rotor, the thing in which the external teeth formed in an outer peripheral surface are formed every 10 degrees may be applied. In this case, frequency division is performed in the engine ECU 6 to generate output pulses every 30 ° CA.

  The cam angle sensor 77 is disposed in the vicinity of the intake camshaft 32, and is used as a cylinder discrimination sensor by outputting a pulse signal corresponding to the compression top dead center (TDC) of the first cylinder, for example. . That is, the cam angle sensor 77 outputs a pulse signal for each rotation of the intake camshaft 32. As an example of the cam angle detection method by the cam angle sensor 77, external teeth are formed at one place on the outer peripheral surface of the rotor integrally formed with the intake camshaft 32, and the external teeth are opposed to each other by an electromagnetic pickup. The cam angle sensor 77 is arranged, and when the external teeth pass in the vicinity of the cam angle sensor 77 as the intake cam shaft 32 rotates, the cam angle sensor 77 generates an output pulse. . Since this rotor rotates at a half rotational speed of the crankshaft 15, an output pulse is generated every time the crankshaft 15 rotates 720 °. In other words, an output pulse is generated each time a specific cylinder reaches the same stroke (for example, when the first cylinder reaches compression top dead center).

  The knock sensor 78 is a vibration sensor that detects the vibration of the engine transmitted to the cylinder block 11 by a piezoelectric element type (piezo element type) or an electromagnetic type (magnet, coil).

  The intake pressure sensor 79 is provided in the surge tank 34, detects the pressure in the intake passage 3 (intake pipe pressure), and transmits the intake pressure signal to the engine ECU 6.

  And engine ECU6 performs various control of the engine 1 by controlling each part, such as the igniter 21, the injector 5, and the throttle motor 36a, based on the output signal of said various sensors 70-79,46,47.

  As an example, the basic control of the ignition timing of the ignition plug 2 by the igniter 21 is that the ignition timing is advanced and corrected so that the ignition timing approaches MBT (Minimum Spark Advance for Best Torque). When knocking is detected by the knock sensor 78, control is performed such that knocking is canceled by correcting the retard of the ignition timing.

As fuel injection control of the injector 5, the target air-fuel ratio is calculated based on the engine load, the engine speed, etc., and the target air-fuel ratio is obtained based on the intake air amount detected by the air flow meter 72. Control of the fuel injection amount (control of the valve opening time of the injector 5) is performed. For example, the oxygen concentration in the exhaust gas is calculated based on the outputs of the A / F sensor 46 and the O ₂ sensor 47 arranged in the exhaust passage 4, and the actual air-fuel ratio obtained from the calculated oxygen concentration is the target air-fuel ratio. “Air-fuel ratio feedback control” is executed to control the fuel injection amount injected from the injector 5 to the intake port 12a so as to match (for example, the theoretical air-fuel ratio). Details of the “air-fuel ratio feedback control” will be described later.

Further, the engine ECU 6 performs the deterioration determination operation of the start catalyst 44 by “catalyst deterioration determination active control” using the O ₂ sensor 47 and the responsiveness diagnosis operation of the A / F sensor 46 by “A / F active control”. Execute. These controls will also be described later.

  Further, as drive control of the throttle motor 36a, the engine ECU 6 that has received the accelerator opening signal based on the opening of the accelerator pedal 35 operated by the driver sends a control signal to the throttle motor 36a, and the throttle position sensor 75 Based on the feedback signal of the opening of the throttle valve 36, the throttle valve 36 is controlled to an appropriate opening. Thereby, the amount of air introduced into the cylinder of the engine 1 is controlled.

-Air-fuel ratio feedback control-
Next, an outline of the air-fuel ratio feedback control will be described.

In the air-fuel ratio feedback control in the present embodiment, the main feedback control for bringing the exhaust air-fuel ratio upstream of the start catalyst 44 close to the stoichiometric air-fuel ratio (target air-fuel ratio) based on the output of the A / F sensor 46, Based on the output of the O ₂ sensor 47, the sub feedback control for compensating for the deviation of the main feedback control is executed in combination.

  In the main feedback control, the increase / decrease in the fuel injection amount from the injector 5 is adjusted so that the exhaust air / fuel ratio detected based on the output of the A / F sensor 46 matches the stoichiometric air / fuel ratio. More specifically, if the detected exhaust air-fuel ratio is richer than the stoichiometric air-fuel ratio, the fuel injection amount is adjusted to decrease. Conversely, if the exhaust air-fuel ratio is leaner than the stoichiometric air-fuel ratio, the fuel injection amount is adjusted. Is adjusted to increase.

  According to this main feedback control, ideally, the air-fuel ratio of the exhaust gas flowing into the start catalyst 44 can be maintained at the stoichiometric air-fuel ratio. If the state is strictly maintained, the stored oxygen amount of the start catalyst 44 is maintained at a substantially constant amount, so that exhaust gas containing unpurified components is completely prevented from flowing downstream. can do.

  However, the output of the A / F sensor 46 includes a certain amount of error. In addition, there is some variation in the injection characteristics of the injector 5. Therefore, in practice, it is difficult to strictly control the exhaust air / fuel ratio upstream of the start catalyst 44 to the stoichiometric air / fuel ratio only by executing the main feedback control. Further, in the engine 1, control is performed to intentionally deviate the exhaust air-fuel ratio from the stoichiometric air-fuel ratio, such as fuel increase or fuel cut. When these controls are performed, the start catalyst 44 may be in a state in which oxygen is completely desorbed or in a state where oxygen is fully occluded, and the state in which unpurified components are likely to flow out downstream. It becomes.

  For the reasons described above, even if the main feedback control is being performed, exhaust gas containing unpurified components may flow out downstream of the start catalyst 44. That is, even if the main feedback control is being performed, the exhaust air-fuel ratio upstream of the start catalyst 44 may be biased to the rich side or the lean side as a whole, and as a result, HC and CO are downstream of the start catalyst 44. Rich exhaust gas containing NO or lean exhaust gas containing NOx may flow out.

When such an outflow occurs, the O ₂ sensor 47 generates a rich output or a lean output according to the air-fuel ratio of the exhaust gas. Therefore, in the system of the present embodiment, when a rich output is generated from the O ₂ sensor 47, it can be determined that the exhaust air-fuel ratio upstream of the start catalyst 44 is biased to the rich side as a whole, When the lean output is issued from the O ₂ sensor 47, it can be determined that the exhaust air-fuel ratio upstream of the start catalyst 44 is biased to the lean side as a whole.

In the sub feedback control, control (for example, PID control) for reducing the difference between the output value of the O ₂ sensor 47 and the control target value of the output value is executed. More specifically, a process of correcting the output of the A / F sensor 46 is performed so that the above difference is reduced. When the output of the A / F sensor 46 is corrected as described above, the air-fuel ratio upstream of the start catalyst 44 that is biased toward the rich side or the lean side as a whole is brought close to the stoichiometric air-fuel ratio. As a result, the deviation of the main feedback control is compensated, and a state in which unpurified components are difficult to blow down downstream of the start catalyst 44 is formed. For this reason, it is possible to realize excellent emission characteristics. In addition, the correction speed of the sub feedback control is lower than the correction speed by the main feedback control in order to cope with the change with time. Further, the learning value obtained by the learning operation of the sub-feedback control is stored in the backup RAM 64 and is updated every time the learning operation is performed.

  As described above, the sum of the correction value corresponding to the short-term change due to the main feedback control and the learning value corresponding to the temporal change due to the sub-feedback control is obtained as the feedback correction amount, and the fuel injection amount is adjusted to increase. Or the weight loss will be adjusted.

-Active control for determining catalyst degradation-
Next, the catalyst deterioration determination active control will be described. The catalyst deterioration determination active control is for determining the deterioration of the start catalyst 44. As the catalyst 44 continues to be used, the catalyst 44 deteriorates due to poisoning by lead or sulfur contained in the fuel or heat applied to the catalyst, and the maximum oxygen storage amount decreases according to the degree of the deterioration. Therefore, if the maximum oxygen storage amount can be calculated and estimated with high accuracy, the deterioration of the catalyst 44 can be determined.

The control performed to calculate the maximum oxygen storage amount of the catalyst 44 is active control for determining catalyst deterioration. In this catalyst deterioration determination active control, when the O ₂ sensor 47 is generating a lean output, the air-fuel ratio (target air-fuel ratio) of the air-fuel mixture supplied to the engine 1 is forcibly made rich (for example, air-fuel ratio 14). .1) and then the O ₂ sensor 47 emits a rich output, the air-fuel ratio (target air-fuel ratio) of the air-fuel mixture supplied to the engine 1 is forcibly set to the lean side (for example, air-fuel ratio 15.1). ). In this way, every time the detection value of the O ₂ sensor 47 is inverted between rich and lean, the target air-fuel ratio of the air-fuel mixture is inverted between the lean side and the rich side. Along with the reversal operation of the target air-fuel ratio, the state in which the start catalyst 44 fully stores oxygen and the state in which the stored oxygen is completely released are repeatedly realized. Therefore, the oxygen storage capacity (maximum) of the start catalyst 44 is integrated by integrating the oxygen amount flowing into the start catalyst 44 within these periods or by integrating the oxygen deficiency in the exhaust gas flowing into the start catalyst 44. The oxygen storage amount Cmax can be obtained by calculation. Based on Cmax calculated by this method, the state of deterioration of the start catalyst 44 can be detected. The above is the active control for catalyst deterioration determination.

-A / F active control-
Next, A / F active control for diagnosing the responsiveness of the A / F sensor 46, which is a characteristic feature of the embodiment, will be described.

  First, the outline of this A / F active control will be described. This A / F active control is executed at a timing different from the catalyst deterioration determination active control described above. As a specific operation, an operation for changing the target air-fuel ratio to the rich side (for example, air-fuel ratio 14.1) and the lean side (for the target air-fuel ratio (for example, theoretical air-fuel ratio = 14.6) before the start of the diagnostic operation) For example, the operation of changing to the air-fuel ratio 15.1) is alternately switched in a short time (for example, 0.6 sec), and the A / F sensor 46 outputs A / F based on the change in the output of the A / F sensor 46 (change in the output voltage). The responsiveness of the F sensor 46 is diagnosed. That is, when the A / F sensor 46 has high responsiveness, as shown by the solid line waveform in FIG. 7, a large amount of change in sensor output per unit time can be obtained, whereas A / F sensor 46 When the responsiveness of the F sensor 46 is insufficient, the change amount of the sensor output per unit time becomes small as shown by the broken line waveform in FIG. By determining these, the responsiveness of the A / F sensor 46 is diagnosed.

  A feature of the present embodiment is that the execution / non-execution (prohibition) of the A / F active control is controlled in accordance with the oxygen concentration atmosphere inside each of the catalysts 44 and 45. In addition, it is also a feature of this embodiment that an air-fuel ratio correction operation is performed so that execution of A / F active control is permitted early in this A / F active control prohibited state. Hereinafter, the procedure of these control operations will be described with reference to the flowcharts of FIGS. The routine shown in FIG. 3 and FIG. 4 is repeatedly executed with a cycle time of, for example, several seconds during the operation of the engine 1.

First, in step ST1, calculation of an estimated value of O ₂ concentration (oxygen concentration) on the downstream side of the underfloor catalyst 45 is performed. Specifically, it is calculated by the following equation (1).

vrro2 = vrro2old + (vro2-vrro2old) / k (ga, oscsc) (1)
Here, vrro2 is an estimated value of O ₂ concentration on the downstream side of the underfloor catalyst 45 (value converted to the output voltage of the O ₂ sensor 47), and vrro2old is the previous value of vrro2 (vrro2 calculated in the routine executed last time). Vro2 is the output voltage value of the O ₂ sensor 47, ga is the amount of intake air per unit time (mass of intake air per second), and oscsc is the estimated value of oxygen storage amount of the start catalyst 44 (oxygen storage amount). Mass) and k (ga, oscsc) are annealing constants determined based on the estimated values of the intake air amount and the oxygen storage amount. The annealing constant is set experimentally or empirically, mapped in advance, and stored in the ROM 62.

Thus, in the calculation operation of the O ₂ concentration estimated value (vrro2) on the downstream side of the underfloor catalyst 45 in step ST1, the output voltage value (vro2) of the O ₂ sensor 47 and the downstream side of the underfloor catalyst 45 are calculated. obtained by the O ₂ concentration preceding value of the estimated values obtained by the smoothing process with respect to the difference between (vrro2old), it is added to the downstream side of the O ₂ concentration previous value of the estimated value of the underfloor catalyst 45 (vrro2old) The obtained value is estimated as the O ₂ concentration (vrro2) on the downstream side of the current underfloor catalyst 45.

  Thereafter, in step ST2, it is determined whether or not an execution condition for A / F active control is satisfied. For example, when all of the following conditions (a) to (d) are satisfied, it is determined that the execution condition for the A / F active control is satisfied.

(A) The engine speed is stable within a predetermined speed range (for example, within a range of 1000 rpm to 4000 rpm),
(B) The fluctuation amount of the intake air amount (ga) per unit time is within a predetermined range, and the intake air amount is stable.
(C) The concentration of the evaporated fuel generated in the fuel tank into the intake system (purge concentration) is below a predetermined amount,
(D) that the fuel injection amount learning operation has been completed;
If the above conditions are satisfied and a Yes determination is made in step ST2, the process proceeds to step ST3, and whether or not the O ₂ atmosphere (oxygen concentration inside the catalyst) of each of the start catalyst 44 and the underfloor catalyst 45 is satisfactory. Is determined (determination operation by the oxygen concentration atmosphere determination means). For example, it is determined whether the oxygen storage amount in each of the start catalyst 44 and the underfloor catalyst 45 is within a range of 40% to 60% with respect to the oxygen storage capacity. This range is the “predetermined oxygen concentration atmosphere in which the oxygen concentration atmosphere inside the catalyst is suitable for purification of exhaust gas” in the present invention.

In this case, whether or not the O ₂ atmosphere inside the start catalyst 44 is good is determined based on the output voltage value (vro2) of the O ₂ sensor 47. Whether the O ₂ atmosphere in the under floor catalyst 45 is good is determined based on the estimated O ₂ concentration (vrro2) on the downstream side of the under floor catalyst 45 calculated in step ST1. Is called. Specifically, when the following formula (2) is established, it is determined that the O ₂ atmosphere inside the start catalyst 44 is good (the atmosphere is in a predetermined oxygen concentration suitable for exhaust gas purification). When the following expression (3) is established, it is determined that the O ₂ atmosphere inside the underfloor catalyst 45 is good (it is in a predetermined oxygen concentration atmosphere suitable for exhaust gas purification). Is done.

VTGT-VDIV1 <vro2 <VTGT + VDIV1 (2)
VTGT-VDIV1 <vrro2 <VTGT + VDIV1 (3)
Here, VTGT is an output target voltage value of the O ₂ sensor 47. VDIV1 is a target convergence determination deviation amount. That is, it is a threshold value for determining whether or not the atmosphere has a predetermined oxygen concentration suitable for exhaust gas purification.

As a specific value of the VTGT, the output target voltage value of the O ₂ sensor 47 is slightly richer than the stoichiometric (theoretical air / fuel ratio = 14.6) (for example, the air / fuel ratio is 14.4). Is set to This makes it possible to particularly increase the NOx purification rate. For example, when the engine 1 is normally driven (when it is not determined whether the O ₂ atmosphere of each of the start catalyst 44 and the underfloor catalyst 45 is good), the output target voltage value of the O ₂ sensor 47 is set. On the other hand, when determining whether or not the O ₂ atmosphere of each of the start catalyst 44 and the underfloor catalyst 45 is good as described above, the output target voltage value of the O ₂ sensor 47 is determined from the stoichiometry. Also, the control performed when the value is slightly switched to the rich side (set to the above VTGT) is performed. Further, in any state, the control may be such that the output target voltage value of the O ₂ sensor 47 is set slightly richer than the stoichiometry.

Thus, both the output voltage value (vro2) of the O ₂ sensor 47 and the estimated O ₂ concentration value (vrro2) on the downstream side of the underfloor catalyst 45 are within a range of ± VDIV1 with respect to the target voltage value (VTGT). If it is within the range, it is determined that the O ₂ atmosphere of each of the start catalyst 44 and the underfloor catalyst 45 is good.

Then, when it is determined that the O ₂ atmosphere of each of the start catalyst 44 and the underfloor catalyst 45 is good and a Yes determination is made in step ST3, the process proceeds to step ST4 and the above-described A / F active control is executed. That is, the operation of changing the target air-fuel ratio to the rich side and the operation of changing to the lean side are alternately switched in a short time with respect to the target air-fuel ratio before the start of the diagnostic operation, and the output change of the A / F sensor 46 Monitor (change in output voltage).

In step ST5, the A / F sensor 46 is responsively diagnosed. That is, when the change amount of the sensor output per unit time is large, it is diagnosed that the A / F sensor 46 has high responsiveness (see the waveform shown by the solid line in FIG. 7). . On the other hand, when the change amount of the sensor output per unit time is small, it is diagnosed that the responsiveness of the A / F sensor 46 is insufficient (see the waveform shown by the broken line in FIG. 7). More specifically, the sensor output change amount is integrated every unit time (for example, every several μsec, and when the integrated value in a predetermined period exceeds a preset threshold value, the A / F sensor 46 is While it is diagnosed that high responsiveness is obtained, if this integrated value does not exceed the threshold value, it is diagnosed that the responsiveness of the A / F sensor 46 is insufficient. The response of the A / F sensor 46 is diagnosed, that is, when the O ₂ atmosphere of each of the start catalyst 44 and the underfloor catalyst 45 is determined to be good, the response of the A / F sensor 46 is diagnosed. Executed.

On the other hand, the start catalyst 44 and the underfloor catalyst 45 one of O ₂ atmosphere of each of the O ₂ atmosphere is not good (deviates to the rich side or the lean side from the range of the threshold value (± VDIV1)) case, or If both the O ₂ atmospheres are not good, the process proceeds to step ST6 (FIG. 4). That is, in this case, the A / F active control is temporarily prohibited, and the process proceeds to the operation after step ST6 (prohibition of the responsiveness diagnostic operation by the diagnostic operation prohibiting means).

  In step ST6, it is determined whether or not both of the following two conditions (e) and (f) are satisfied.

(E) The execution condition of A / F active control has changed from non-satisfied to established,
Downstream of the O ₂ concentration estimated value of (f) underfloor catalyst 45 that (O values converted to the output voltage of the _secondary sensor 47) is less than the output target voltage value of the O ₂ sensor 47 (vrro2 <VTGT), i.e. The O ₂ concentration estimated value (vrro2) is an output on the lean side of the output target voltage value (VTGT) of the O ₂ sensor 47,
If YES is determined in step ST6, the process proceeds to step ST7, and the following two values are set.

・ Status uf = LEAN
・ Count stage = 1
On the other hand, if No is determined in step ST6, the process proceeds to step ST8, and it is determined whether or not both of the following two conditions (e) and (g) are satisfied.

(E) The execution condition of A / F active control has changed from non-satisfied to established,
(G) downstream of the O ₂ concentration estimated value of underfloor catalyst 45 that (O ₂ values converted to the output voltage of the sensor 47) is the output target voltage value or more of the O ₂ sensor 47 (vrro2 ≧ VTGT), i.e. or the O ₂ concentration estimated value (vrro2) coincides with the output target voltage value of the O ₂ sensor 47 (VTGT), than the output target voltage value of the O ₂ sensor 47 is on the rich side output,
If YES is determined in step ST8, the process proceeds to step ST9 and the following two values are set.

・ Status uf = RICH
・ Count stage = 1
If the determination in step ST8 is No, this routine is terminated as it is. For example, this is a case where the execution condition of the A / F active control is still not established.

  After each value (status uf and count stage) is set in step ST7 or step ST9, the process proceeds to step ST10. In step ST10, it is determined whether one of the following two conditions (h) and (i) is satisfied.

(H) status uf = LEAN and vrro2 ≧ VTGT−VDIV2
(I) status uf = RICH and vrro2 <VTGT + VDIV2
Here, VDIV2 the underfloor downstream of the O ₂ concentration estimated value of the catalyst 45 (Vrro2) improves the start catalyst 44 inside the O ₂ atmosphere from the control to improve the internal O ₂ atmosphere predetermined state (underfloor catalyst 45 This is a threshold value for determining whether or not it has been improved to a state in which the transition to the control to be performed is permitted), and is set to a value smaller than VDIV1 (target convergence determination deviation amount).

That is, when the A / F active control is not executed because the O ₂ atmosphere inside the underfloor catalyst 45 is lean, when the above condition (h) is satisfied, the downstream side of the underfloor catalyst 45 is The estimated O ₂ concentration (vrro2) is judged to have approached the deviation of VDIV2 to the lean side with respect to the output target voltage value of the O ₂ sensor 47, and the O ₂ atmosphere inside the underfloor catalyst 45 is A / F active control. Is determined to be permitted.

Similarly, when the A / F active control is not executed because the O ₂ atmosphere inside the underfloor catalyst 45 is rich, the downstream side of the underfloor catalyst 45 is satisfied when the above condition (i) is satisfied. The estimated O ₂ concentration (vrro2) of the O ₂ sensor is judged to have approached the deviation of VDIV2 to the rich side with respect to the output target voltage value of the O ₂ sensor 47, and the O ₂ atmosphere inside the underfloor catalyst 45 is A / F active. It is determined that the control is permitted.

  The threshold (VDIV2) is set in advance so that the above determination is made. For example, the VDIV2 is set as a voltage value “0.1V”. This value is not limited to this.

In the situation where the A / F active control is not executed because the O ₂ atmosphere inside the underfloor catalyst 45 is lean or rich, when the determination of step ST10 is performed first, the air-fuel ratio correction operation described later is still in progress. Since it is not executed, neither of the above conditions (h) and (i) is established, and a NO determination is made in step ST10 and the process proceeds to step ST11.

In step ST11, it is determined whether the current status uf is “LEAN” (status uf = LEAN). That is, it is determined whether or not the A / F active control is not performed because the O ₂ atmosphere inside the underfloor catalyst 45 is lean. If this determination is Yes, the process proceeds to step ST12, where the target air-fuel ratio is shifted to the rich side (air-fuel ratio correction operation to the rich side by the air-fuel ratio correction means). More specifically, to correct only the rich side correction amount (VR = 0.05 V) target value of the output voltage value of the O ₂ sensor 47 (vro2tgt) output target voltage value of the O ₂ sensor 47 with respect to (VTGT) (Vro2tgt = VTGT + VR).

On the other hand, if NO is determined in step ST11, that is, if the A / F active control is not performed because the O ₂ atmosphere inside the underfloor catalyst 45 is rich, step Moving to ST13, the target air-fuel ratio is shifted to the lean side (air-fuel ratio correction operation to the lean side by the air-fuel ratio correcting means). More specifically, the correction target value of the output voltage value of the O ₂ sensor 47 (vro2tgt) lean correction amount to the output target voltage value of the O ₂ sensor 47 (VTGT) only (VL = 0.10 V) (Vro2tgt = VTGT-VL).

  In such an air-fuel ratio correction operation, the lean correction amount (0.10 V) is set larger than the rich correction amount (0.05 V). This is because the situation in which the oxygen storage amount inside the underfloor catalyst 45 is insufficient is quickly resolved, and the exhaust purification by the oxidation reaction inside the underfloor catalyst 45 is promoted.

In this way, the air-fuel ratio correction operation for improving the O ₂ atmosphere inside the underfloor catalyst 45 is performed, and the air-fuel ratio correction operation is performed while maintaining the count stage at “1” until a Yes determination is made in step ST10. Continue. By shifting the target air-fuel ratio by the air-fuel ratio correction operation as described above, the exhaust air-fuel ratio is also changed by performing the main feedback control and the sub-feedback control described above (so-called modern feedback control), and the underfloor catalyst The O ₂ atmosphere inside 45 is improved.

When it is determined that one of the above conditions (h) and (i) is satisfied and the O ₂ atmosphere inside the underfloor catalyst 45 is in a state in which A / F active control is permitted, Yes is determined in step ST10, and the process proceeds to step ST14. In step ST14, the count stage is changed to "2" and the process proceeds to step ST15.

In step ST15, it causes the output voltage target value of the O ₂ sensor 47 (vro2tgt) matches the target output voltage value of the O ₂ sensor 47 (VTGT). As a result, the O ₂ atmosphere inside the start catalyst 44 is improved.

As described above, when the A / F active control is not executed, the air-fuel ratio correction operation is performed, and the improvement of the O ₂ atmosphere inside the underfloor catalyst 45 and the improvement of the O ₂ atmosphere inside the start catalyst 44 are sequentially performed. If the O ₂ atmosphere (oxygen concentration inside the catalyst) of each of the start catalyst 44 and the underfloor catalyst 45 becomes good, a Yes determination is made in step ST3, and execution of the above-described A / F active control (step ST4), responsiveness diagnosis of the A / F sensor 46 is performed by executing this A / F active control (step ST5).

As described above, in the present embodiment, when at least one of the internal start catalyst 44 in an O ₂ atmosphere and underfloor catalyst 45 inside the O ₂ atmosphere is not in a predetermined O ₂ atmosphere suitable for purification of exhaust gas The A / F active control and the accompanying responsiveness diagnosis operation of the A / F sensor 46 are prohibited. For this reason, when the A / F active control is performed in the lean state of the O ₂ atmosphere inside the catalysts 44 and 45, NOx is released into the atmosphere, and the O ₂ atmosphere inside the 44 and 45 is When the A / F active control is performed in the rich state, a situation in which HC and CO are released into the atmosphere can be avoided, and deterioration of exhaust emission can be prevented.

In this embodiment, when the responsiveness diagnosis operation is prohibited due to the lean O ₂ atmosphere inside the catalysts 44 and 45, the air-fuel ratio correction operation is performed so that the air-fuel ratio is shifted to the rich side. (Change of target air-fuel ratio) is performed, and when the responsiveness diagnosis operation is prohibited because the O ₂ atmosphere inside the catalysts 44 and 45 is rich, the air-fuel ratio is made to shift to the lean side. A fuel ratio correction operation (change of the target air-fuel ratio) is performed. For this reason, the O ₂ atmosphere inside the catalysts 44 and 45 is improved at an early stage, and the O ₂ atmosphere inside the catalysts 44 and 45 becomes a predetermined amount suitable for exhaust gas purification within a relatively short time. Reach O ₂ atmosphere. Thereby, the prohibition state of the responsiveness diagnostic operation is released, and the execution of the responsiveness diagnostic operation of the A / F sensor 46 is permitted. By thus performing the control operation for positively improving the O ₂ atmosphere inside the catalysts 44 and 45 in this way, after the execution condition of the responsiveness diagnosis operation of the A / F sensor 46 is established, within a short period of time. It becomes possible to execute a responsiveness diagnosis operation.

Furthermore, in this embodiment, after the O ₂ atmosphere inside the underfloor catalyst 45 is improved, the O ₂ atmosphere inside the start catalyst 44 is improved. The reason is that since the start catalyst 44 is close to the combustion chamber 16, the response of the change in the O ₂ atmosphere when the air-fuel ratio correction operation is performed is high. On the other hand, since the underfloor catalyst 45 is far from the combustion chamber 16, the response of the change in the O ₂ atmosphere when the air-fuel ratio correction operation is performed is lower than that of the start catalyst 44. Therefore, after the improved O ₂ atmosphere in the start catalyst 44, than was internal underfloor catalyst 45 from the O ₂ atmosphere improving operation, in the course control for the underfloor catalyst 45, more responsive start catalyst There is a possibility that the O ₂ atmosphere inside 44 will deviate from an appropriate state, and when A / F active control is executed, there is a possibility that exhaust emission will deteriorate due to this misalignment. For this reason, in this embodiment, the air-fuel ratio correction operation for improving the O ₂ atmosphere of the underfloor catalyst 45 is performed first. As a result, both the start catalyst 44 and the underfloor catalyst 45 start the A / F active control in a state where the O ₂ atmosphere is improved, and it is possible to reliably avoid the deterioration of the exhaust emission.

-Experimental example-
The result of the experiment example performed for confirming the effect of embodiment mentioned above is demonstrated. In this experimental example, the control operation according to the above embodiment is executed in a state where the O ₂ atmosphere inside the start catalyst 44 and the underfloor catalyst 45 is set to the rich side or the lean side. The first experimental example, when the start catalyst 44 inside the O ₂ atmosphere and underfloor catalyst 45 together inside the O ₂ atmosphere LEAN, second experimental example, and a RICH an O ₂ atmosphere in the start catalyst 44 When the O ₂ atmosphere inside the underfloor catalyst 45 is LEAN, the third experimental example is the fourth when the O ₂ atmosphere inside the start catalyst 44 is LEAN and the O ₂ atmosphere inside the underfloor catalyst 45 is RICH. examples of the experiment is the case where both the RICH the start catalyst 44 inside the O ₂ atmosphere and underfloor catalyst 45 inside the O ₂ atmosphere.

Non-execution / execution switching state of A / F active control, change in output voltage value of O ₂ sensor 47, change in estimated O ₂ concentration downstream of underfloor catalyst, change in count stage in the above experimental example , Status uf, and A / F active control execution condition unsatisfied / satisfied switching state are shown in the timing charts of FIGS. 5 and 6, respectively. FIG. 5 is a timing chart when A / F active control is executed in the LEAN state of the underfloor catalyst (when the left side is the start catalyst 44 when LEAN is used, and the right side is when the start catalyst 44 is RICH), FIG. 6 is a timing chart when A / F active control is executed in the RICH state of the underfloor catalyst (when the start catalyst 44 is LEAN on the left side and the start catalyst 44 is RICH on the right side).

In these timing charts, at timing T1, the A / F active control execution condition is satisfied (Yes is determined in step ST2), the count stage is set to “1”, and the status uf is set to “LEAN” or “RICH”. It is the set timing. Timing T2 is timing when the count stage is changed to “2” in accordance with the improvement of the O ₂ atmosphere inside the underfloor catalyst 45. Furthermore, timing T3 is a timing at which the O ₂ atmosphere inside each catalyst 44, 45 is improved and A / F active control is started.

As can be seen from these figures, after the O ₂ concentration estimated value (vrro2) on the downstream side of the underfloor catalyst 45 is improved by the air-fuel ratio correcting operation, the output voltage value (vro2) of the O ₂ sensor 47 is changed. improvement is performed, thereby, underfloor catalyst 45 inside the O ₂ atmosphere improved start improvement of the catalyst 44 inside the O ₂ atmosphere is performed in order, then, become a / F active control is executed ing. Thereby, deterioration of exhaust emission is prevented.

-Other embodiments-
In the embodiment described above, the case where the present invention is applied to the four-cylinder gasoline engine 1 mounted on an automobile has been described. The present invention is applicable not only to automobiles but also to engines used for other purposes. Also, the number of cylinders and the engine type (separate types such as in-line type, V type, and horizontally opposed type) are not particularly limited.

Further, in the above embodiment, as an operation for recognizing the O ₂ atmosphere inside the underfloor catalyst 45, the estimated O ₂ concentration on the downstream side of the underfloor catalyst 45 is calculated. The present invention is not limited to this, and the oxygen storage amount inside the underfloor catalyst 45 may be directly recognized. For example, the oxygen storage amount inside the underfloor catalyst 45 is recognized using the above-described active control for catalyst deterioration determination.

Further, in the above embodiment, when the A / F active control is prohibited, first, the improvement operation of the O ₂ atmosphere inside the underfloor catalyst 45 is started, and after that, the O 2 inside the start catalyst 44 is started. I had to to start the ₂ atmosphere of improving operations. The present invention is not limited to this, and in the middle of the operation of improving the O ₂ atmosphere in the underfloor catalyst 45, the operation of starting the O ₂ atmosphere in the start catalyst 44 is started. the operation to improve the ₂ atmosphere one which concurrently is also included in the scope of the technical idea.

1 is a system configuration diagram illustrating a schematic configuration of an engine according to an embodiment. It is a figure which shows the outline of the control block of an engine. It is a flowchart figure which shows a part of operation | movement procedure of A / F active control. It is a flowchart figure which shows a part of other operation | movement procedure of A / F active control. When A / F active control is executed in the LEAN state of the underfloor catalyst, the non-execution / execution state of A / F active control, the change in the output voltage value of the O ₂ sensor, the downstream of the underfloor catalyst FIG. 5 is a timing chart showing a change state of an estimated O ₂ concentration, a transition of count stage, a change of status uf, and a state where A / F active control execution conditions are not established / established. When A / F active control is executed in the RICH state of the underfloor catalyst, the non-execution / execution state of A / F active control, the change in the output voltage value of the O ₂ sensor, the downstream of the underfloor catalyst FIG. 5 is a timing chart showing a change state of an estimated O ₂ concentration, a transition of count stage, a change of status uf, and a state where A / F active control execution conditions are not established / established. The alternate long and short dash line indicates the waveform indicating the change in the target air-fuel ratio, the solid line indicates the sensor output waveform when the A / F sensor has high responsiveness, and the broken line indicates that the A / F sensor has sufficient responsiveness. It is a figure which shows the sensor output waveform when not being performed.

Explanation of symbols

1 engine (internal combustion engine)
44 Start catalyst (upstream catalyst)
45 Underfloor catalyst (downstream catalyst)
46 A / F sensor (air-fuel ratio sensor)
47 O ₂ sensor (oxygen sensor)

Claims (4)

An air-fuel ratio sensor that outputs a signal corresponding to the oxygen concentration of the exhaust gas upstream of the catalyst is provided upstream of the catalyst that purifies the exhaust gas of the internal combustion engine. In an air-fuel ratio control apparatus for an internal combustion engine that forcibly switches the air-fuel ratio of the engine alternately between the rich side and the lean side,
Oxygen concentration atmosphere determination means for determining whether the oxygen concentration atmosphere inside the catalyst is a predetermined oxygen concentration atmosphere suitable for exhaust gas purification;
When the oxygen concentration atmosphere determination means determines that the oxygen concentration atmosphere inside the catalyst is not a predetermined oxygen concentration atmosphere suitable for exhaust gas purification, the diagnostic operation prohibition prohibits the responsiveness diagnostic operation of the air-fuel ratio sensor. And an air-fuel ratio control device for an internal combustion engine. In the air-fuel ratio control apparatus for an internal combustion engine according to claim 1,
When it is determined by the oxygen concentration atmosphere determination means that the oxygen concentration atmosphere inside the catalyst is not a predetermined oxygen concentration atmosphere suitable for exhaust gas purification, the oxygen concentration atmosphere is determined as predetermined oxygen suitable for exhaust gas purification. Air-fuel ratio correction means for performing an air-fuel ratio correction operation so as to approach the concentration atmosphere,
When the oxygen concentration atmosphere in the catalyst becomes a predetermined oxygen concentration atmosphere suitable for exhaust gas purification by the air-fuel ratio correcting operation by the air-fuel ratio correcting means, the response of the air-fuel ratio sensor by the diagnostic operation prohibiting means An air-fuel ratio control apparatus for an internal combustion engine, wherein the prohibition state of the performance diagnosis operation is canceled and execution of the response diagnosis operation of the air-fuel ratio sensor is permitted. In the air-fuel ratio control apparatus for an internal combustion engine according to claim 2,
The catalyst is composed of an upstream catalyst and a downstream catalyst connected in series, and the air-fuel ratio correcting means converts the oxygen concentration atmosphere inside the downstream catalyst into a predetermined oxygen concentration suitable for exhaust gas purification. After the air-fuel ratio correction operation is performed so as to approach the atmosphere, and the oxygen concentration atmosphere inside the downstream catalyst becomes a predetermined oxygen concentration atmosphere suitable for exhaust gas purification, the oxygen concentration atmosphere inside the upstream catalyst is changed to An air-fuel ratio control apparatus for an internal combustion engine, characterized in that an air-fuel ratio correction operation is performed so as to approach an atmosphere of a predetermined oxygen concentration suitable for exhaust gas purification. In the air-fuel ratio control apparatus for an internal combustion engine according to claim 1 or 2,
The catalyst is composed of an upstream catalyst and a downstream catalyst connected in series, and a rich signal or a lean signal corresponding to the oxygen concentration of the exhaust gas is output between the upstream catalyst and the downstream catalyst. An oxygen sensor is provided,
Both the signal corresponding to the oxygen concentration of the exhaust gas on the downstream side of the upstream catalyst outputted from the oxygen sensor and the estimated oxygen concentration value on the downstream side of the downstream catalyst estimated based on this signal are both within a predetermined range. If the oxygen concentration atmosphere in the upstream catalyst and the downstream catalyst are both in a predetermined oxygen concentration atmosphere suitable for exhaust gas purification, the air-fuel ratio sensor responsiveness diagnosis operation is executed. An air-fuel ratio control apparatus for an internal combustion engine characterized by comprising:

Images