JP2010138705A - Air-fuel ratio control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an air-fuel ratio control device properly controlling an air-fuel ratio according to an activated state of a catalyst in relation to air-fuel ratio feedback control. <P>SOLUTION: For the air-fuel ratio feedback control performing main feedback control by output from an air-fuel ratio sensor 76 upstream of a three way catalyst 42 and sub-feedback control by output from an oxygen sensor 77 downstream of the three way catalyst 42, the more catalyst temperature is low, the more a stoichiometric demand level is varied to a rich side. Further, the more an increment per unit time of a throttle valve opening degree is large, the more the stoichiometric demand level is varied to the rich side. Thereby, even in a situation where the temperature of the three way catalyst 42 is low and the activity is insufficient, air-fuel feedback control suitable for the situation can be carried out. <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 an improvement for optimizing the air-fuel ratio in an internal combustion engine having an exhaust system provided with exhaust gas sensors (air-fuel ratio sensor and oxygen concentration sensor) on the upstream side and downstream side of the catalyst, respectively.

  Conventionally, as disclosed, for example, in Patent Documents 1 to 5 below, in an exhaust system of an automobile engine, an air-fuel ratio sensor (hereinafter referred to as an A / F sensor) is provided upstream of a catalyst (for example, a three-way catalyst). In some cases, an oxygen concentration sensor (hereinafter also simply referred to as an oxygen sensor) is provided on the downstream side of the catalyst.

  With this configuration, the fuel injection amount is feedback-controlled based on the output signal of the A / F sensor so that the air-fuel ratio of the exhaust gas flowing into the catalyst becomes the target air-fuel ratio (for example, the theoretical air-fuel ratio). Yes (main feedback control).

  In addition to this main feedback control, control is performed to correct the output target value of the A / F sensor based on the output signal of the oxygen sensor, and an error in the output signal due to degradation of the A / F sensor is corrected. Yes (sub feedback control).

  By executing such feedback control (generally called cosmic control), the fuel injection amount from the injector is adjusted so that the air-fuel ratio of the intake air becomes the target air-fuel ratio (for example, the theoretical air-fuel ratio), and the exhaust gas Emissions are improved.

Further, an air-fuel ratio change request value (generally called a stoichiometric request level: an output indicating a request to shift the air-fuel ratio to a rich side or a lean side with respect to stoichiometry, which is an output signal of the oxygen sensor when performing the sub-feedback control. The meaning of the signal is changed not only according to the oxygen concentration on the downstream side of the catalyst but also according to the intake air amount (engine load) of the engine. Specifically, when the throttle valve opening increases and the intake air amount increases (engine load increases), the oxygen sensor stoichiometric request level is changed according to the increase in the intake air amount. To do. That is, the stoichiometric request level of the oxygen sensor is changed so that the air-fuel ratio at which engine torque corresponding to the engine load is obtained, and the output target value of the A / F sensor is corrected to the rich side.
JP 2002-332896 A JP-A-7-305647 JP-A-8-303234 JP 2000-97081 A JP-A-8-144802

  However, the required stoichiometric level of the conventional oxygen sensor is merely changed according to the intake air amount (engine load) of the engine. In other words, assuming that the catalyst is in an active state, that is, the exhaust gas purification performance (purification performance of HC, CO, NOx, etc.) of the catalyst is sufficiently exerted, the stoichiometric request level corresponding to the intake air amount is set. I was just asking for it.

  For this reason, if the catalyst is not in an active state or if the activity is insufficient, that is, in a situation where exhaust purification performance is not sufficiently obtained, there is a possibility that an appropriate air-fuel ratio cannot be obtained. That is, when the activity of the catalyst is insufficient, the exhaust purification performance is inferior compared to when the catalyst is in an active state. For this reason, the NOx emission amount tends to increase, for example, on the downstream side of the catalyst, but the above-described feedback control so far has not been sufficient to cope with such a situation.

  Also, in each of the above patent documents, there is no consideration regarding obtaining an appropriate air-fuel ratio when the activity of the catalyst is insufficient, and appropriate air-fuel ratio feedback control according to the active state of the catalyst can be performed. It was not. For example, Patent Document 1 discloses that the feedback control constant is corrected according to the catalyst temperature, and Patent Document 2 discloses that the update speed of the feedback control constant is changed according to the catalyst temperature. None of them has the technical idea for optimizing the stoichiometric requirement level.

  The present invention has been made in view of the above points, and an object of the present invention relates to air-fuel ratio feedback control. An air-fuel ratio control apparatus capable of performing appropriate air-fuel ratio control according to the active state of a catalyst is provided. It is to provide.

-Principle of solving the problem-
The solution principle of the present invention taken in order to achieve the above object is that when performing air-fuel ratio feedback control, the catalyst temperature is used as a parameter, and the air-fuel ratio change request value (stoichiometric request level) of the exhaust gas sensor on the downstream side of the catalyst is set. Thus, feedback control according to the active state of the catalyst is performed.

-Solution-
Specifically, according to the present invention, exhaust gas sensors are respectively provided on the upstream side and the downstream side of the catalyst provided in the exhaust passage of the internal combustion engine, and the output of each exhaust gas sensor is set so that the air-fuel ratio of the internal combustion engine becomes the target air-fuel ratio. An air-fuel ratio control apparatus for an internal combustion engine that performs air-fuel ratio feedback control for controlling the air-fuel ratio by a feedback correction amount based on the value is assumed. The air-fuel ratio control apparatus changes the air-fuel ratio change request value of the exhaust gas sensor downstream of the catalyst in accordance with the temperature of the catalyst, and the air-fuel ratio feedback based on the changed air-fuel ratio change request value of the exhaust gas sensor. Sensor required value changing means for performing control is provided.

  With this specific matter, when performing air-fuel ratio feedback control based on the output values of the exhaust gas sensors upstream and downstream of the catalyst, the temperature of the catalyst is detected or estimated. Then, the air-fuel ratio change request value (corresponding to the above-mentioned stoichiometric request level) of the exhaust gas sensor downstream of the catalyst is changed according to the temperature of the catalyst, and the feedback correction amount is set so that the air-fuel ratio of the internal combustion engine becomes the target air-fuel ratio. Control the air / fuel ratio. As a result, even when the catalyst temperature is low and the catalyst activity is insufficient, it is possible to perform air-fuel ratio feedback control suitable for it. For example, even in a situation where the amount of NOx emission increases due to insufficient exhaust purification performance of the catalyst, NOx is reduced by enriching the air-fuel ratio change request value of the exhaust gas sensor downstream of the catalyst. Air-fuel ratio feedback control that can reduce the emission amount can be realized.

  Specific examples of the air-fuel ratio feedback control for changing the air-fuel ratio change request value of the exhaust gas sensor on the downstream side of the catalyst according to the catalyst temperature include the following methods. First, in the air-fuel ratio feedback control, the output target value of the exhaust gas sensor upstream of the catalyst is changed according to the air-fuel ratio change request value of the exhaust gas sensor downstream of the catalyst. Then, the sensor request value changing means requests the air-fuel ratio change of the exhaust gas sensor on the downstream side of the catalyst so that the output target value of the exhaust gas sensor on the upstream side of the catalyst shifts to the rich side as the temperature of the catalyst decreases. The value is changed.

  If the temperature of the catalyst is low and not fully activated, there is a risk that the amount of NOx discharged to the downstream side of the catalyst will increase. In such a case, in this solution, the air-fuel ratio change request value of the exhaust gas sensor on the downstream side of the catalyst is changed so that the output target value of the exhaust gas sensor on the upstream side of the catalyst is shifted to the rich side. Thereby, the amount of oxygen in the exhaust gas is relatively reduced, and the NOx emission amount can be suppressed. As a result, exhaust emission can be improved even under conditions where the catalyst is not sufficiently activated.

  More specifically, a configuration for changing the air-fuel ratio change request value of the exhaust gas sensor by the sensor request value changing means is as follows. First, there is provided a storage means for storing a plurality of maps corresponding to the temperature of the catalyst for changing the air-fuel ratio change request value of the exhaust gas sensor on the downstream side of the catalyst according to the intake air amount of the internal combustion engine. And the said sensor request value change means selects one map corresponding to the temperature of a catalyst among the several maps memorize | stored in the said memory | storage means, According to the temperature of the said catalyst, and the said intake air amount The air-fuel ratio change request value of the exhaust gas sensor on the downstream side of the catalyst is acquired, and the air-fuel ratio change request value of the exhaust gas sensor on the downstream side of the catalyst is changed.

  This solution is to change the air-fuel ratio change request value of the exhaust gas sensor on the downstream side of the catalyst using the catalyst temperature and the intake air amount as parameters. In other words, a three-dimensional control configuration in which these three components are correlated to improve exhaust emission and obtain the torque of the internal combustion engine in accordance with the intake air amount. In addition, when the temperature of the catalyst changes with the continuous operation of the internal combustion engine, the sensor request value changing means changes the air-fuel ratio change request value of the exhaust gas sensor on the downstream side of the catalyst while switching the map to be selected. Will do.

  Moreover, in addition to the change of the air-fuel ratio change request value of the exhaust gas sensor according to the above solution (change according to the catalyst temperature), the air-fuel ratio change request value may be changed as follows. That is, the sensor request value changing means further changes the air-fuel ratio change request value of the exhaust gas sensor downstream of the catalyst according to the amount of change per unit time of the opening degree of the throttle valve provided in the intake system of the internal combustion engine. It is comprised so that it may do.

  In this case, specifically, as the amount of increase in the throttle valve opening per unit time is larger, the output target value of the exhaust gas sensor on the upstream side of the catalyst is shifted to the rich side so that the downstream side of the catalyst is shifted to the rich side. The request value for changing the air-fuel ratio of the exhaust gas sensor is changed.

  For example, when the amount of increase in the throttle valve opening per unit time increases with the driver's accelerator operation or the like, the combustion chamber temporarily becomes lean, and the NOx emission tends to increase. In this case, in this solution, the air-fuel ratio change request value of the exhaust gas sensor on the downstream side of the catalyst is changed so that the output target value of the exhaust gas sensor on the upstream side of the catalyst is shifted to the rich side. Thereby, the NOx emission amount is suppressed. In other words, even when the catalyst temperature is low and the catalyst is not sufficiently activated and the amount of intake air increases, the amount of NOx emission is particularly likely to increase. It becomes possible.

  In the present invention, when the air-fuel ratio feedback control is performed based on the output values of the exhaust gas sensors provided upstream and downstream of the catalyst, the air-fuel ratio change request value of the exhaust gas sensor on the downstream side of the catalyst is changed using the catalyst temperature as a parameter. Thus, feedback control according to the active state of the catalyst is performed. For this reason, even in a situation where the activity of the catalyst is insufficient, it is possible to perform air-fuel ratio control suitable for it, and it is possible to improve exhaust emission.

  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-
FIG. 1 is a diagram showing a schematic configuration of an engine 1 and its intake / exhaust system according to the present embodiment. In FIG. 1, only the configuration of one cylinder of the engine 1 is shown.

  The engine 1 in this embodiment is, for example, a 4-cylinder gasoline engine, and includes a piston 12 that forms a combustion chamber 11 and a crankshaft 13 that is an output shaft. The piston 12 is connected to the crankshaft 13 via a connecting rod 14, and the reciprocating motion of the piston 12 is converted into rotation of the crankshaft 13 by the connecting rod 14.

  A signal rotor 15 having a plurality of protrusions (teeth) 16 on the outer peripheral surface is attached to the crankshaft 13. A crank position sensor (engine speed sensor) 71 is disposed near the side of the signal rotor 15. The crank position sensor 71 is, for example, an electromagnetic pickup, and generates a pulsed signal (output pulse) corresponding to the protrusion 16 of the signal rotor 15 when the crankshaft 13 rotates.

  A water temperature sensor 72 for detecting the engine water temperature (cooling water temperature) is disposed in the cylinder block 17 of the engine 1.

  A spark plug 2 is disposed in the combustion chamber 11 of the engine 1. The ignition timing of the spark plug 2 is adjusted by the igniter 21. The igniter 21 is controlled by an engine ECU (Electronic Control Unit) 6.

  An intake passage 3 and an exhaust passage 4 are connected to the combustion chamber 11 of the engine 1. An intake valve 31 is provided between the intake passage 3 and the combustion chamber 11. By opening and closing the intake valve 31, the intake passage 3 and the combustion chamber 11 are communicated or blocked. An exhaust valve 41 is provided between the exhaust passage 4 and the combustion chamber 11, and the exhaust passage 4 and the combustion chamber 11 are communicated or blocked by opening and closing the exhaust valve 41. The opening / closing drive of the intake valve 31 and the exhaust valve 41 is performed by each rotation of an intake camshaft and an exhaust camshaft (both not shown) to which the rotation of the crankshaft 13 is transmitted.

  An air cleaner 32, a hot-wire air flow meter 73, an intake air temperature sensor 74 (built in the air flow meter 73), and an electronically controlled throttle valve 33 that adjusts the intake air amount of the engine 1 are disposed in the intake passage 3. ing. The throttle valve 33 is driven by a throttle motor 34. The opening degree of the throttle valve 33 is detected by a throttle opening degree sensor 75.

A three-way catalyst 42 is disposed in the exhaust passage 4 of the engine 1. The three-way catalyst 42 has an O ₂ storage function (oxygen storage function) for storing (occluding) oxygen. Even if the air-fuel ratio shifts from the stoichiometric air-fuel ratio to a certain extent by this oxygen storage function, 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 catalyst 42 increase, the three-way catalyst 42 occludes part of the oxygen, thereby reducing and purifying NOx. Promote. On the other hand, when the air-fuel ratio of the engine 1 becomes rich and the exhaust gas flowing into the three-way catalyst 42 contains a large amount of HC and CO, the three-way catalyst 42 releases oxygen molecules stored therein, Oxygen molecules are given to these HC and CO to promote oxidation and purification.

  An air-fuel ratio sensor (A / F sensor) 76 is disposed in the exhaust passage 4 upstream of the three-way catalyst 42. The air-fuel ratio sensor (exhaust gas sensor) 76 is, for example, a limit current type oxygen concentration sensor, and is configured to generate an output voltage corresponding to the air-fuel ratio over a wide air-fuel ratio region. That is, as shown in FIG. 3, the voltage signal vabyfs corresponding to the air-fuel ratio (A / F) on the upstream side of the three-way catalyst 42 is output. As apparent from FIG. 3, the air-fuel ratio sensor 76 can accurately detect an air-fuel ratio over a wide range.

An oxygen sensor (O ₂ sensor) 77 is disposed in the exhaust passage 4 on the downstream side of the three-way catalyst 42. As this oxygen sensor (exhaust gas sensor) 77, for example, an electromotive force type (concentration cell type) oxygen concentration sensor is applied, and as shown in FIG. 4, a voltage Voxs that changes suddenly at the stoichiometric air-fuel ratio is output. ing. More specifically, the oxygen sensor 77 is, for example, approximately 0.1 (V) when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio, and approximately 0.1 when the air-fuel ratio is richer than the stoichiometric air-fuel ratio. When the air-fuel ratio is 9 (V) and the stoichiometric air-fuel ratio, a voltage of approximately 0.5 (V) is output.

  The signals generated by the air-fuel ratio sensor 76 and the oxygen sensor 77 are each A / D converted and then input to the engine ECU 6.

  A fuel injection injector 35 is disposed in the intake passage 3. Fuel of a predetermined pressure is supplied from the fuel tank to the injector 35 by a fuel pump, and the fuel is injected into the intake passage 3. This injected fuel is mixed with intake air to form an air-fuel mixture and introduced into the combustion chamber 11 of the engine 1. The air-fuel mixture (fuel + air) introduced into the combustion chamber 11 undergoes a compression stroke of the engine 1 and is then ignited by the spark plug 2 to burn and explode. The piston 12 is reciprocated by the combustion / explosion of the air-fuel mixture in the combustion chamber 11 to rotate the crankshaft 13.

-Description of control block-
The operating state of the engine 1 is controlled by the engine ECU 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 nonvolatile memory that stores data to be saved when the engine 1 is stopped.

  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.

  In addition to the crank position sensor 71, water temperature sensor 72, air flow meter 73, intake air temperature sensor 74, throttle opening sensor 75, air-fuel ratio sensor 76, oxygen sensor 77, the external input circuit 65 includes an accelerator opening sensor 78, A cam angle sensor 79, a knock sensor 7A, and the like are connected. On the other hand, the external output circuit 66 is connected to a throttle motor 34 for driving the throttle valve 33, the injector 35, the igniter 21 and the like.

  The crank position sensor 71 is disposed in the vicinity of the crankshaft 13 as described above, and detects the rotation angle (crank angle CA) and the rotation speed (engine speed Ne) of the crankshaft 13.

  The water temperature sensor 72 detects the temperature of the cooling water flowing in the water jacket 17a formed in the cylinder block 17, and transmits the cooling water temperature signal to the engine ECU 6.

  The air flow meter 73 detects the intake air amount and transmits the intake air amount signal to the engine ECU 6.

  The intake air temperature sensor 74 is provided integrally with the air flow meter 73, detects the intake air temperature, and transmits the intake air temperature signal to the engine ECU 6.

  The throttle opening sensor 75 detects the opening of the throttle valve 33 and transmits the throttle opening signal to the engine ECU 6.

  The air-fuel ratio sensor 76 generates an output voltage corresponding to the air-fuel ratio of the exhaust discharged from the combustion chamber 11 (the exhaust on the upstream side of the three-way catalyst 42), and transmits the output voltage signal to the engine ECU 6.

  The oxygen sensor 77 generates an output voltage corresponding to the oxygen concentration of the exhaust gas downstream of the three-way catalyst 42 and transmits the output voltage signal to the engine ECU 6.

  The accelerator opening sensor 78 detects the opening (operation amount) of the accelerator pedal operated by the driver, and transmits the opening signal to the engine ECU 6.

  The cam angle sensor 79 is disposed in the vicinity of the intake camshaft 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 79 outputs a pulse signal for each rotation of the intake camshaft. As an example of a cam angle detection method using this cam angle sensor, external teeth are formed at one location on the outer peripheral surface of the rotor integrally formed with the intake camshaft, and the external teeth are opposed to each other by an electromagnetic pickup. A cam angle sensor 79 is arranged, and when the external teeth pass near the cam angle sensor 79 as the intake cam shaft rotates, the cam angle sensor 79 generates an output pulse. Since this rotor rotates at half the rotational speed of the crankshaft 13, an output pulse is generated every time the crankshaft 13 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 7A is a vibration sensor that detects the vibration of the engine transmitted to the cylinder block 17 by a piezoelectric element type (piezo element type) or an electromagnetic type (magnet, coil).

  Then, the engine ECU 6 executes various controls of the engine 1 based on the detection signals of the various sensors described above. For example, the oxygen concentration in the exhaust gas is calculated based on the outputs of the air-fuel ratio sensor 76 and the oxygen sensor 77 arranged in the exhaust passage 4 of the engine 1, and the actual air-fuel ratio obtained from the calculated oxygen concentration is the target air “Air-fuel ratio feedback control” is executed to control the fuel injection amount injected from the injector 35 into the intake passage 3 so as to coincide with the fuel ratio (for example, the theoretical air-fuel ratio).

-Air-fuel ratio feedback control-
Next, a specific operation procedure of the air-fuel ratio feedback control will be described.

  Before describing the control (control for changing the air-fuel ratio change request value (stoichiometric request level) of the oxygen sensor 77 in accordance with the temperature of the three-way catalyst 42), the basic operation of the air-fuel ratio feedback control will be described. Will be described.

The three-way catalyst 42 functions to oxidize unburned components (HC, CO) and reduce nitrogen oxides (NOx) at the same time when the air-fuel ratio is substantially the stoichiometric air-fuel ratio. Further, as described above, the three-way catalyst 42 has a function of storing oxygen (oxygen storage function, O ₂ storage function), and the oxygen storage function shifts the air-fuel ratio from the stoichiometric air-fuel ratio to some extent. However, HC, CO and NOx can be purified. That is, when the air-fuel ratio of the engine 1 is lean and the gas flowing into the three-way catalyst 42 contains a large amount of NOx, the three-way catalyst 42 deprives the NOx of oxygen molecules and occludes these oxygen molecules and stores NOx. Reduce, thereby purifying NOx. Further, when the air-fuel ratio of the engine 1 becomes rich and the gas flowing into the three-way catalyst 42 contains a large amount of HC and CO, the three-way catalyst 42 gives the stored oxygen molecules to the HC and CO and oxidizes them. This purifies HC and CO.

  Therefore, in order for the three-way catalyst 42 to efficiently purify a large amount of continuously flowing HC and CO, the three-way catalyst 42 must store a large amount of oxygen. In order to efficiently purify a large amount of NOx that flows in, the three-way catalyst 42 needs to be in a state where it can sufficiently store oxygen. As is clear from the above, the purification capacity of the three-way catalyst 42 depends on the maximum amount of oxygen that can be stored by the three-way catalyst 42 (maximum oxygen storage amount).

  On the other hand, the three-way catalyst 42 deteriorates due to poisoning due to lead, sulfur, etc. contained in the fuel, or heat applied to the three-way catalyst 42, and accordingly, the maximum oxygen storage amount gradually decreases. In this way, even when the maximum oxygen storage amount is reduced, in order to maintain the emission satisfactorily, the air-fuel ratio of the gas discharged from the three-way catalyst 42 is in a state that is very close to the stoichiometric air-fuel ratio. Need to control.

  Therefore, the output Voxs of the oxygen sensor 77 (that is, downstream of the three-way catalyst 42) is set so that the output of the oxygen sensor 77, which is one of the state quantities related to the exhaust of the engine 1, becomes the target value Voxsref that substantially corresponds to the theoretical air-fuel ratio. The air-fuel ratio of the engine 1 is feedback-controlled according to the air-fuel ratio).

  In this embodiment, the air-fuel ratio is feedback controlled according to the output vabyfs of the air-fuel ratio sensor 76, and this is referred to as main feedback control. On the other hand, air-fuel ratio feedback control based on the output of the oxygen sensor 77 is referred to as sub-feedback control.

  In the main feedback control, the increase / decrease in the fuel injection amount from the injector 35 is adjusted so that the exhaust air / fuel ratio detected based on the output of the air / fuel ratio sensor 76 matches the target air / fuel ratio (for example, the theoretical air / fuel ratio). The More specifically, if the detected exhaust air-fuel ratio is richer than the target air-fuel ratio, the fuel injection amount is adjusted to decrease. Conversely, if the exhaust air-fuel ratio is leaner than the target air-fuel ratio, the fuel injection amount is adjusted. Is adjusted to increase.

  According to the main feedback control, ideally, the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 42 can be maintained at the stoichiometric air-fuel ratio. If the state is strictly maintained, the stored oxygen amount of the three-way catalyst 42 is maintained at a substantially constant amount, so that exhaust gas containing unpurified components can flow completely downstream. Can be blocked.

  However, the output of the air-fuel ratio sensor 76 includes a certain amount of error. In addition, there is some variation in the injection characteristics of the injector 35. Therefore, in reality, it is difficult to strictly control the exhaust air / fuel ratio upstream of the three-way catalyst 42 to the stoichiometric air / fuel ratio only by executing the main feedback control.

  For the above reasons, even if the main feedback control is being performed, exhaust gas containing unpurified components may flow out downstream of the three-way catalyst 42. That is, even if the main feedback control is being performed, the exhaust air-fuel ratio upstream of the three-way catalyst 42 may be biased to the rich side or the lean side as a whole, and as a result, the HC Rich exhaust gas containing CO and CO or lean exhaust gas containing NOx may flow out.

  When such an outflow occurs, the oxygen sensor 77 generates a rich output or a lean output according to the air-fuel ratio of the exhaust gas. Therefore, in the system of this embodiment, when a rich output is generated from the oxygen sensor 77, it can be determined that the exhaust air-fuel ratio upstream of the three-way catalyst 42 is biased to the rich side as a whole, When a lean output is generated from the oxygen sensor 77, it can be determined that the exhaust air-fuel ratio upstream of the three-way catalyst 42 is biased to the lean side as a whole.

  In the sub-feedback control, when the output of the oxygen sensor 77 becomes a value representing an air-fuel ratio leaner than the stoichiometric air-fuel ratio, the deviation between the output Voxs of the oxygen sensor 77 and the target value Voxsref substantially corresponding to the stoichiometric air-fuel ratio is proportional / integrated. Processing (PID processing) is performed to determine the sub feedback correction amount vafsfb. Then, the output vabyfs of the air-fuel ratio sensor 76 is corrected by this sub-feedback correction amount vafsfb, so that the actual air-fuel ratio of the engine 1 appears to be leaner than the detected air-fuel ratio of the air-fuel ratio sensor 76. And the feedback control is performed so that the corrected apparent air-fuel ratio becomes the target air-fuel ratio (the target air-fuel ratio of the engine 1, here the theoretical air-fuel ratio).

  Similarly, when the output of the oxygen sensor 77 becomes a value representing an air / fuel ratio richer than the stoichiometric air / fuel ratio, the deviation between the output Voxs of the oxygen sensor 77 and the target value Voxsref substantially corresponding to the stoichiometric air / fuel ratio is proportionally / integrated ( Sub-feedback correction amount vafsfb is obtained by PID processing). Then, the output vabyfs of the air-fuel ratio sensor 76 is corrected by the sub-feedback correction amount vafsfb, so that the actual air-fuel ratio of the engine 1 appears to be richer than the detected air-fuel ratio of the air-fuel ratio sensor 76. And the feedback control is performed so that the corrected apparent air-fuel ratio becomes the target air-fuel ratio (the target air-fuel ratio of the engine 1, here the theoretical air-fuel ratio).

  As described above, the air-fuel ratio downstream of the three-way catalyst 42 is matched with the target air-fuel ratio (substantially theoretical air-fuel ratio) at the same site.

  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. The learning value (sub feedback correction amount) 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 so that the fuel injection amount is adjusted to increase or The weight loss will be adjusted.

  In particular, in the sub feedback control according to the present embodiment, the learning gain at the time of the sub feedback learning operation is changed during the learning operation. Specifically, the learning gain is gradually set smaller in the order of the initial period of the learning operation, the learning operation completion determination period, and the learning operation completion determination. For this reason, even if the learning value is greatly deviated, the learning value can be greatly corrected by the large learning gain in the initial learning period (the learning speed can be increased). By setting a small value, it is possible to avoid that the learning value is greatly shifted again. The initial period of the learning operation, that is, the period during which the learning gain is set large is set shorter than the other period, that is, the period during which the learning gain is set small. The above is the basic operation of the air-fuel ratio feedback control in the present embodiment.

-Air-fuel ratio control according to catalyst temperature-
(Determining the stoichiometric requirement level according to the catalyst temperature)
Next, control for changing the air-fuel ratio change request value (stoichiometric request level) of the oxygen sensor 77 according to the temperature of the three-way catalyst 42, which is a feature of the present embodiment, will be described. This control changes the stoichiometric request level of the oxygen sensor 77 (value for setting the output target value of the air-fuel ratio sensor 76) in accordance with the temperature of the three-way catalyst 42 in the sub feedback control described above. . This will be specifically described below.

  The ROM 62 of the engine ECU 6 stores a plurality of stoichiometric request level acquisition maps corresponding to the temperature of the three-way catalyst 42. FIG. 5 shows the relationship between the temperature of the three-way catalyst 42 and the corresponding stoichiometric demand level acquisition map. FIG. 5 shows a case where three types of stoichiometric request level acquisition maps are stored in the ROM (storage means) 62 in accordance with the temperature of the three-way catalyst 42. That is, the stoichiometric demand level acquisition map (map A in the figure) on the left side in FIG. 5 is when the temperature of the three-way catalyst 42 is relatively low (for example, when it is 300 ° C. or less: at the initial stage of engine start and in the light load operation state). This corresponds to a case where the activation of the three-way catalyst 42 is insufficient. Further, the right stoichiometric demand level acquisition map (map C in the figure) in FIG. 5 corresponds to a case where the temperature of the three-way catalyst 42 is relatively high (for example, 600 ° C. or higher). Furthermore, the center stoichiometric demand level acquisition map (map B in the figure) in FIG. 5 corresponds to the case where the temperature of the three-way catalyst 42 is in the intermediate temperature range of the above-mentioned two. The number of the stoichiometric request level acquisition maps and the temperature ranges to which the maps correspond are not limited to those described above.

  Each stoichiometric requirement level acquisition map defines the relationship between the intake air amount (corresponding to the engine load) and the stoichiometric requirement level corresponding thereto. That is, the stoichiometric request level of the oxygen sensor 77 is changed so as to obtain an air-fuel ratio at which an engine torque is output according to the engine load due to an increase in the throttle valve opening accompanying an increase in the accelerator pedal depression amount of the driver, This is a map for correcting the output target value of the air-fuel ratio sensor 76. Each stoichiometric request level acquisition map is given a map value (stoichiometric request level) corresponding to each intake air amount so that the stoichiometric request levels are different from each other even if the intake air amount is the same.

  In the sub feedback control, one of three types of stoichiometric request level acquisition maps is selected according to the temperature of the three-way catalyst 42. Then, according to the selected stoichiometric demand level acquisition map, the stoichiometric demand level is read (obtained) according to the intake air amount, and this stoichiometric demand level is determined based on the output value of the oxygen sensor 77 (the output target of the air-fuel ratio sensor 76). Output value for determining the value) (operation for changing the air-fuel ratio change request value (stoichiometric request level) by the sensor request value changing means).

  Specifically, as the stoichiometric demand level acquisition map is selected when the temperature of the three-way catalyst 42 is low, the rich side value is acquired as the stoichiometric demand level even when the intake air amount is the same. It has become. In other words, as the temperature of the three-way catalyst 42 is lower, the stoichiometric request level of the oxygen sensor 77 is changed so that the output target value of the air-fuel ratio sensor 76 is shifted to the rich side even with the same intake air amount. I have to. For example, the output target value of the air-fuel ratio sensor 76 is changed from a value corresponding to stoichiometric (air-fuel ratio 14.6) to a value corresponding to rich (for example, air-fuel ratio 14.4).

  That is, in the present embodiment, as the relationship between the intake air amount and the stoichiometric request level, a plurality of “catalyst temperature axes” corresponding to the temperature of the three-way catalyst 42 are provided, and the air-fuel ratio is set according to the temperature of the three-way catalyst 42. By switching the stoichiometric demand level acquisition map used for feedback control, the relationship between the intake air amount and the stoichiometric demand level is changed. As a result, the lower the temperature of the three-way catalyst 42, the stoichiometric requirement level is determined as the rich side value even with the same intake air amount.

  These stoichiometric demand level acquisition maps are set in advance by experiments, simulations, or the like so that the discharge amount of HC, CO, NOx is sufficiently reduced according to the intake air amount.

  For example, when the air-fuel ratio is lean and the temperature of the three-way catalyst 42 is low and NOx is likely to be discharged (in the operating state in which NOx emission is confirmed by the above-described experiment or simulation), A stoichiometric demand level acquisition map (map A) corresponding to the catalyst low temperature is selected, and the stoichiometric demand level of the oxygen sensor 77 is set to a rich value so that the output target value of the air-fuel ratio sensor 76 is shifted to the rich side. The Thereby, the NOx emission amount is suppressed. Further, when the air-fuel ratio is rich and the temperature of the three-way catalyst 42 is high and HC and CO are easily discharged (operation in which HC and CO emissions have been confirmed by the above experiments and simulations). In the state), the stoichiometric demand level acquisition map (map C) corresponding to the catalyst high temperature is selected, and the stoichiometric demand level of the oxygen sensor 77 is set to the lean side so that the output target value of the air-fuel ratio sensor 76 is shifted to the lean side. Set to a value. Thereby, the discharge amount of HC and CO is suppressed.

(Correction of stoichiometric requirement level according to throttle valve opening)
Next, correction according to the throttle valve opening of the stoichiometric request level of the oxygen sensor 77 determined according to the temperature of the three-way catalyst 42 as described above will be described.

  For example, when the amount of increase in the opening degree of the throttle valve 33 per unit time increases with the driver's accelerator operation, the interior of the combustion chamber 11 temporarily becomes lean, and the NOx emission tends to increase. For this reason, in this embodiment, if the NOx emission tends to increase in this way, the stoichiometric request level of the oxygen sensor 77 is corrected so that the output target value of the air-fuel ratio sensor 76 is shifted to the rich side. I am doing so. This will be specifically described below.

  FIG. 6A is a diagram showing the relationship between the amount of change in the throttle opening and the correction coefficient for correcting the stoichiometric request level. FIG. 6B is a diagram showing the relationship between the air amount and the corrected stoichiometric request level when the stoichiometric request level is corrected in accordance with the throttle opening change amount.

  As shown in FIG. 6A, when the amount of change in the throttle opening exceeds a predetermined amount, the correction coefficient increases as the amount of change increases. That is, the correction amount with respect to the stoichiometric requirement level determined according to the temperature of the three-way catalyst 42 is increased. The corrected stoichiometric demand level is obtained by an operation such as multiplying the stoichiometric demand level determined according to the temperature of the three-way catalyst 42 by the correction coefficient. Further, the corrected stoichiometric demand level may be obtained according to a map stored in advance (a map for correcting the stoichiometric demand level in accordance with the amount of increase in the opening degree of the throttle valve 33 per unit time).

  The solid line in FIG. 6B shows the relationship between the intake air amount and the stoichiometric request level when the amount of change in throttle opening does not exceed the predetermined amount. The broken line in FIG. 6B shows the relationship between the intake air amount and the stoichiometric request level when the amount of change in the throttle opening exceeds a predetermined amount and the amount of NOx emission increases. Yes. That is, the increase in the NOx emission amount due to the change in the throttle opening exceeding the predetermined amount is suppressed by correcting the stoichiometric request level for correcting the stoichiometric request level to the rich side to the rich side. In addition, the alternate long and short dash line in FIG. 6B shows the case where the amount of intake air and the stoichiometric request level when the amount of change in throttle opening exceeds a predetermined amount and the amount of HC and CO emissions increases. Showing the relationship. That is, an increase in the amount of HC and CO emissions due to the change in the throttle opening exceeding the predetermined amount is suppressed by correcting the stoichiometric request level to the lean side.

  Whether the amount of NOx emissions increases or the amount of HC and CO emissions increases when the amount of change in throttle opening exceeds a predetermined amount has been confirmed in advance by experiments, simulations, etc. When the amount of NOx emission increases according to the state and the amount of change in throttle opening, the stoichiometric request level is corrected to the rich side as shown by the broken line in FIG. When increasing, the stoichiometric requirement level is corrected to the lean side as shown by the alternate long and short dash line in FIG.

(Air-fuel ratio control)
Next, the sub feedback control procedure by setting the stoichiometric request level according to the catalyst temperature and the throttle valve opening as described above will be described with reference to the flowchart of FIG.

  First, in step ST1, the temperature of the three-way catalyst 42 is estimated. This is estimated from the current engine operating condition. Specifically, a catalyst temperature estimation map for estimating the temperature of the three-way catalyst 42 from the engine speed and engine load (throttle valve opening etc.) is stored in the ROM 62, and the catalyst temperature estimation map is The temperature of the three-way catalyst 42 is estimated by fitting the engine speed and the engine load. Further, the temperature estimation operation of the three-way catalyst 42 may be estimated from the exhaust gas temperature. For example, exhaust gas temperature sensors are provided upstream and downstream of the three-way catalyst 42, and the temperature of the three-way catalyst 42 is estimated based on the exhaust gas temperature detected by the exhaust gas temperature sensors. Further, the temperature of the three-way catalyst 42 may be directly detected.

  After estimating (or detecting) the temperature of the three-way catalyst 42 in this way, the process proceeds to step ST2, and the stoichiometric request level based on the catalyst active state is determined. That is, since the active state of the three-way catalyst 42 has a correlation with the temperature, the catalyst active state can be determined from the estimated temperature of the three-way catalyst 42, and the oxygen sensor 77 corresponding to the catalyst active state can be determined. Determine the output (stoichiometric request level). Specifically, as described with reference to FIG. 5, the stoichiometric request level acquisition map corresponding to the current temperature of the three-way catalyst 42 is selected. Then, according to the selected stoichiometric demand level acquisition map, the stoichiometric demand level is read (obtained) according to the intake air amount, and the value is determined as the stoichiometric demand level of the oxygen sensor 77.

  Thereafter, the process proceeds to step ST3, where it is determined whether or not the increase amount per unit time of the opening degree of the throttle valve 33 exceeds a predetermined amount in accordance with the driver's accelerator operation. That is, as described with reference to FIG. 6, it is determined whether or not the throttle opening change amount exceeds the predetermined amount and the exhaust emission is deteriorated.

  If the increase amount per unit time of the opening degree of the throttle valve 33 exceeds a predetermined amount, and YES is determined in step ST3, the process proceeds to step ST4. In step ST4, according to FIG. 6A, a correction coefficient for correcting the stoichiometric request level is obtained according to the amount of increase in the opening degree of the throttle valve 33 per unit time, which is shown in FIG. 6B. Thus, the stoichiometric request level is corrected. Thereafter, the process proceeds to step ST5.

  Further, if the increase amount per unit time of the opening degree of the throttle valve 33 does not exceed the predetermined amount and NO is determined in step ST3, the process proceeds to step ST5 without correcting the stoichiometric request level in step ST4. .

  In step ST5, the stoichiometric request level set as described above is output from the oxygen sensor 77, and sub-feedback control for shifting the output target value of the air-fuel ratio sensor 76 to the rich side or the lean side is executed accordingly. In other words, the output target value of the air-fuel ratio sensor 76 is set to the rich side when the NOx emission amount increases, and the output target value of the air-fuel ratio sensor 76 is set to the lean side when the HC and CO emission amounts increase. Will be.

  As described above, in the present embodiment, by setting the stoichiometric request level according to the catalyst temperature and the throttle valve opening and executing the sub-feedback control, the temperature of the three-way catalyst 42 is low and the activity is insufficient. Even under certain circumstances, it becomes possible to implement air-fuel ratio feedback control suitable for it. As a result, air-fuel ratio feedback control that can reduce the NOx emission amount downstream of the three-way catalyst 42 even in a situation where the NOx emission amount increases due to insufficient exhaust purification performance. Can be achieved, and exhaust emission can be improved.

-Other embodiments-
In the embodiment described above, the case where the present invention is applied to the automobile four-cylinder gasoline engine 1 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.

  In the above-described embodiment, the case where the present invention is applied to the case where the learning gain at the time of the sub-feedback learning operation is changed during the learning operation is described. The present invention can also be applied to an apparatus that performs a sub-feedback learning operation without changing the gain.

  In the above embodiment, the exhaust gas sensor upstream of the three-way catalyst 42 is the air-fuel ratio sensor 76, and the exhaust gas sensor downstream of the three-way catalyst 42 is the oxygen sensor 77. The present invention is not limited to this, and an oxygen sensor may be applied as an upstream exhaust gas sensor, or an air-fuel ratio sensor may be applied as a downstream exhaust gas sensor.

  Further, in the above embodiment, the stoichiometric request level determined according to the temperature of the three-way catalyst 42 is corrected according to the increase amount per unit time of the opening degree of the throttle valve 33. The present invention is not limited to this, and the stoichiometric request level is determined only in accordance with the temperature of the three-way catalyst 42 without performing correction according to the amount of increase in the opening degree of the throttle valve 33 per unit time. May be.

  In addition, in the above-described embodiment, when the stoichiometric request level is corrected according to the increase amount per unit time of the opening degree of the throttle valve 33, the correction amount is increased as the intake air amount is increased (see FIG. 6 (b)). The present invention is not limited to this, and the stoichiometric requirement level may be corrected with a uniform correction amount according to the increase amount per unit time of the opening degree of the throttle valve 33 regardless of the intake air amount. That is, the stoichiometric requirement level is corrected by translating the straight line in FIG.

It is a figure which shows schematic structure of the engine which concerns on embodiment, and its intake / exhaust system. It is a block diagram which shows the control system of an engine. It is a figure which shows the relationship between the output voltage of an air fuel ratio sensor, and an air fuel ratio. It is a figure which shows the relationship between the output voltage of an oxygen sensor, and an air fuel ratio. It is a figure which shows the relationship between a catalyst temperature and the stoichiometric request level acquisition map selected according to it. FIG. 6A is a diagram showing the relationship between the throttle opening change amount and the correction coefficient for the stoichiometric request level, and FIG. 6B is a diagram showing the relationship between the air amount and the corrected stoichiometric request level. . It is a flowchart figure which shows the procedure of the sub feedback control which concerns on embodiment.

Explanation of symbols

1 engine (internal combustion engine)
3 Intake passage 33 Throttle valve 4 Exhaust passage 42 Three-way catalyst 62 ROM (storage means)
76 Air-fuel ratio sensor (upstream exhaust gas sensor)
77 Oxygen sensor (downstream exhaust gas sensor)

Claims (5)

Exhaust gas sensors are provided on the upstream side and the downstream side of the catalyst provided in the exhaust passage of the internal combustion engine, respectively, so that the air-fuel ratio of the internal combustion engine becomes empty by the feedback correction amount based on the output value of each exhaust gas sensor. In an air-fuel ratio control apparatus for an internal combustion engine that performs air-fuel ratio feedback control for controlling the fuel ratio,
A sensor request value change for changing the air-fuel ratio change request value of the exhaust gas sensor downstream of the catalyst according to the temperature of the catalyst and performing the air-fuel ratio feedback control based on the changed air-fuel ratio change request value of the exhaust gas sensor An air-fuel ratio control apparatus for an internal combustion engine, characterized by comprising means. In the air-fuel ratio control apparatus for an internal combustion engine according to claim 1,
The air-fuel ratio feedback control is to change an output target value of the exhaust gas sensor on the upstream side of the catalyst in accordance with an air-fuel ratio change request value of the exhaust gas sensor on the downstream side of the catalyst,
The sensor request value changing means changes the air-fuel ratio change request value of the exhaust gas sensor on the downstream side of the catalyst so that the output target value of the exhaust gas sensor on the upstream side of the catalyst is shifted to the rich side as the temperature of the catalyst is lower. An air-fuel ratio control apparatus for an internal combustion engine, characterized in that the structure is changed. In the air-fuel ratio control apparatus for an internal combustion engine according to claim 1 or 2,
The map for changing the air-fuel ratio change request value of the exhaust gas sensor on the downstream side of the catalyst according to the intake air amount of the internal combustion engine includes a plurality of storage means stored corresponding to the temperature of the catalyst,
The sensor request value changing means selects one map corresponding to the temperature of the catalyst from among a plurality of maps stored in the storage means, so that the downstream side of the catalyst corresponding to the catalyst temperature and the intake air amount is selected. An air-fuel ratio control apparatus for an internal combustion engine, characterized in that an air-fuel ratio change request value of an exhaust gas sensor on a side is acquired and an air-fuel ratio change request value of an exhaust gas sensor on the downstream side of the catalyst is changed. In the air-fuel ratio control apparatus for an internal combustion engine according to claim 1, 2, or 3,
The sensor request value changing means further changes the air-fuel ratio change request value of the exhaust gas sensor on the downstream side of the catalyst according to the amount of change per unit time of the opening degree of the throttle valve provided in the intake system of the internal combustion engine. An air-fuel ratio control apparatus for an internal combustion engine, characterized in that it is configured. In the air-fuel ratio control apparatus for an internal combustion engine according to claim 2,
The sensor request value changing means shifts the output target value of the exhaust gas sensor on the upstream side of the catalyst to the rich side as the increase amount per unit time of the opening degree of the throttle valve provided in the intake system of the internal combustion engine increases. Thus, the air-fuel ratio control apparatus for an internal combustion engine, wherein the air-fuel ratio change request value of the exhaust gas sensor on the downstream side of the catalyst is further changed.

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