JP2880886B2 - Air-fuel ratio control device for internal combustion engine - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control device for an internal combustion engine, and more particularly, to an air-fuel ratio of an internal combustion engine for feedback-controlling a detection value of an exhaust gas concentration sensor disposed in an exhaust passage of the internal combustion engine to a target air-fuel ratio. It relates to a control device.

[0002]

2. Description of the Related Art Conventionally, an exhaust gas concentration sensor (hereinafter referred to as a "LAF sensor") having an output characteristic substantially proportional to an exhaust gas concentration is provided in an exhaust passage of an engine, and the LAF sensor is provided.
An air-fuel ratio control device that feedback-controls an output value of a sensor to a target air-fuel ratio is well known.

[0003] The L used in such an air-fuel ratio control device is
As the AF sensor, for example, an oxygen pump element and a battery element are formed by providing an electrode pair on each of two plate-shaped oxygen ion conductive solid electrolyte materials, and each of one electrode surface of the oxygen pump element and the battery element Constitutes a part of a gas diffusion chamber, the gas diffusion chamber communicates with a gas to be measured through an introduction hole, and the other electrode surface of the battery element faces the air chamber.

In such a LAF sensor, the voltage generated by the battery element is compared with a predetermined reference voltage so that the oxygen concentration in the gas diffusion chamber is always maintained at a predetermined concentration (for example, 0). A pump current is supplied to the electrode surface of the pump element, and the pump current value is output as a signal proportional to the oxygen concentration via an amplifier circuit. As a pump current detection system, a current detection resistor connected in series with the oxygen pump element is used, and a voltage across the current detection resistor is extracted as a voltage representing a pump current value.

[0005] Such a LAF sensor has an oxygen concentration detection characteristic in which the pump current IP changes linearly in the rich and lean regions with respect to the stoichiometric air-fuel ratio. The detection of the air-fuel ratio is performed using the relationship between the pump current value and the air-fuel ratio as described above. However, the air-fuel ratio is actually determined from the voltage output from the detection system including the current detection resistor as described above. Is determined.

That is, from the pump current detection system, an output voltage VA having an output characteristic substantially proportional to the exhaust gas concentration is obtained.
F and a center voltage VA which is a voltage value corresponding to the target air-fuel ratio.
FCENT is output. The air-fuel ratio of the air-fuel mixture can be calculated by detecting the deviation between the detected output voltage VAF and the center voltage VAFCENT, and the air-fuel ratio feedback control is performed based on the calculation result.

However, in the above-mentioned method using only the LAF sensor, the target air-fuel ratio is set to the stoichiometric air-fuel ratio (A / F =
When the air-fuel ratio is set to 14.7) and the feedback control of the air-fuel ratio is performed, the air-fuel ratio of the air-fuel mixture is changed due to an error in the output value caused by variation or deterioration of the amplifier circuit connected to the LAF sensor. In some cases, it is practically difficult to converge on the stoichiometric air-fuel ratio. Therefore, the feedback control is performed by setting the target air-fuel ratio to a value slightly deviating from the stoichiometric air-fuel ratio. That is, in the above-described feedback control method, since the actual air-fuel ratio of the air-fuel mixture is feedback-controlled to a value slightly deviating from the stoichiometric air-fuel ratio, the best exhaust gas characteristics may not be obtained. Therefore, as a technique for solving such inconvenience, while an LAF sensor is provided upstream of a catalyst device provided in an exhaust passage of an engine, an O2 sensor whose output signal is inverted near a target air-fuel ratio (stoichiometric air-fuel ratio) is used. A configuration provided on the downstream side of the catalyst device has already been proposed (for example, Japanese Patent Application Laid-Open No. 2-674).
No. 43).

[0008] This publication discloses that when an error occurs in the output value of the LAF sensor due to variation or deterioration of the amplifier circuit or the sensor itself, the detection current (the above-described pump current) is considered due to the output characteristics of the LAF sensor. Paying attention to the point where the slope from the stoichiometric air-fuel ratio to the atmosphere changes in the relationship between the IP and the air-fuel ratio, learning and correcting the slope of the output characteristic of the upstream LAF sensor based on the output value of the downstream O2 sensor. ,
It is intended to control the target air-fuel ratio so that it always becomes the stoichiometric air-fuel ratio.

[0009]

SUMMARY OF THE INVENTION However, LAF
Even in the device disclosed in the above publication that learns and corrects the inclination of the output characteristic of the sensor, the center voltage VAFCENT of the LAF sensor is changed due to variations and deterioration of the amplifier circuit and the sensor itself, as in the conventional method using only the LAF sensor. Cannot be accurately grasped. For this reason, the control amount of the air-fuel ratio of the air-fuel mixture cannot be accurately obtained, so that it is difficult to always maximize the purification efficiency of the catalyst device, and there is a problem that the exhaust gas characteristics deteriorate.

The present invention has been made in view of the above-mentioned conventional problems, and has enabled high-precision air-fuel ratio control so as to maximize the purification efficiency of a catalyst device, and improved the exhaust gas characteristics of an internal combustion engine. The purpose is to provide.

[0011]

In order to achieve the above object, the present invention provides a catalyst device disposed in an exhaust passage of an internal combustion engine for purifying harmful components in exhaust gas, and a catalyst device provided upstream of the catalyst device. A first exhaust concentration sensor disposed in the exhaust passage, having an output characteristic substantially proportional to the exhaust gas concentration, operating state detecting means for detecting an operating state of the engine, and Target air-fuel ratio calculating means for calculating a target air-fuel ratio of the air-fuel mixture supplied to the engine;
A second exhaust concentration sensor disposed in the exhaust passage downstream of the catalyst device and having an output signal inverted near a target air-fuel ratio; and a target air-fuel ratio based on an output value of the second exhaust concentration sensor. A target air-fuel ratio corrector that corrects the target air-fuel ratio calculated by the calculator; and a mixture supplied to the engine according to the corrected target air-fuel ratio and an output value of the first exhaust gas concentration sensor. In the air-fuel ratio control device for an internal combustion engine that controls the air-fuel ratio of the gas, the output value of the second exhaust gas concentration sensor maximizes the conversion efficiency of the catalyst device.
An output value correcting means for correcting a center value corresponding to a target air-fuel ratio which is an output value of the first exhaust gas concentration sensor when the value is within the range .

[0012]

According to the present invention having the above-described structure, the output value correcting means sets the first exhaust gas concentration sensor to the first value when the output value of the second exhaust gas concentration sensor is in a range where the conversion efficiency of the catalyst device is maximized. Since the center voltage corresponding to the target air-fuel ratio, which is the output value of the exhaust gas concentration sensor, is corrected, the variation in the output characteristics of the first exhaust gas concentration sensor is corrected, and the air-fuel ratio control can always be performed with high accuracy.
Exhaust gas characteristics can be improved.

[0013]

Embodiments of the present invention will be described below in detail with reference to the drawings.

FIG. 1 is an overall configuration diagram showing an embodiment of an air-fuel ratio control device for an internal combustion engine according to the present invention.

In FIG. 1, reference numeral 1 denotes a DOHC in-line four-cylinder internal combustion engine (hereinafter, simply referred to as "engine") provided with a pair of intake valves and exhaust valves (not shown) for each cylinder. A throttle body 3 is provided in the middle of an intake pipe 2 of the engine 1, and a throttle valve 3 ′ is disposed inside the throttle body 3. A throttle valve opening (θTH) sensor 4 is connected to the throttle valve 3 ′, and outputs an electric signal according to the opening of the throttle valve 3 ′ to output an electronic control unit (hereinafter referred to as “ECU”) 5. To supply.

The fuel injection valve 6 is provided for each cylinder in the middle of the intake pipe 2 and between the engine 1 and the throttle valve 3 '.
It is electrically connected to U5, and the valve opening time of fuel injection is controlled by a signal from the ECU5.

A branch pipe 7 is provided downstream of the throttle valve 3 ′ of the intake pipe 2, and an absolute pressure (PBA) sensor 8 is attached to a tip of the branch pipe 7. The PBA sensor 8 is electrically connected to the ECU 5, and
The absolute pressure PBA is converted into an electric signal by the PBA sensor 8 and supplied to the ECU 5.

An intake air temperature (TA) sensor 9 is mounted on the pipe wall of the intake pipe 2 on the downstream side of the branch pipe 7, and the intake air temperature TA detected by the TA sensor 9 is converted into an electric signal to be supplied to the ECU 5. Supplied to

An engine coolant temperature (TW) sensor 10 composed of a thermistor or the like is inserted into a cylinder peripheral wall of the cylinder block of the engine 1 filled with coolant, and the engine coolant temperature TW detected by the TW sensor 10 is converted into an electric signal. The converted data is supplied to the ECU 5.

An engine speed (NE) sensor 11 and a cylinder discrimination (CYL) sensor 12 are mounted around a camshaft or a crankshaft (not shown) of the engine 1.

The NE sensor 11 outputs a signal pulse (hereinafter referred to as a "TDC signal pulse") at a predetermined crank angle position every time the crankshaft of the engine 1 rotates by 180 degrees, and CY is output.
The L sensor 12 outputs a TDC signal pulse at a predetermined crank angle position of a specific cylinder, and these TDC signal pulses are supplied to the ECU 5.

The ignition plug 13 of each cylinder of the engine 1
It is electrically connected to the ECU 5, and the ignition timing is controlled by the ECU 5.

A catalyst device (three-way catalyst) 15 is interposed in the exhaust pipe 14 of the engine 1 and purifies harmful components such as HC, CO and NOx in the exhaust gas by the catalyst device 15. Done.

In the middle of the exhaust pipe 14 and upstream and downstream of the catalyst device 15, a wide area oxygen concentration sensor (hereinafter referred to as "LAF sensor") 16 and an oxygen concentration sensor (hereinafter referred to as "O2 sensor"). ) 17 are provided respectively.

As will be described in detail later, the LAF sensor 16 has a pair of upper and lower battery elements and an oxygen pump element attached to predetermined positions of a sensor element made of zirconia solid electrolyte (ZrO ₂ ) or the like. The sensor element is electrically connected to the amplifier circuit. The LAF sensor 16 outputs an output voltage VAF substantially proportional to the oxygen concentration in the exhaust gas passing through the inside of the sensor element and a center voltage VAFCENT corresponding to the target air-fuel ratio, and outputs the electric signal to the ECU 5. Supply.

The O2 sensor 17 has a sensor element similar to the LAF sensor 16 and has a zirconia solid electrolyte (ZrO
₂ ), whose electromotive force changes rapidly before and after the stoichiometric air-fuel ratio. At the stoichiometric air-fuel ratio, the output signal is inverted from a lean signal to a rich signal or from a rich signal to a lean signal. That is, the output signal of the O2 sensor 17 has a high level on the rich side of the exhaust gas and has a low level on the lean side, and supplies the output signal to the ECU 5.

An atmospheric pressure (PA) sensor 18 is provided at an appropriate position in the engine 1 to detect the atmospheric pressure PA, and supplies an electric signal to the ECU 5.

The ECU 5 has an input circuit 5a having a function of shaping the input signal waveforms from the various sensors described above, correcting the voltage level to a predetermined level, and converting an analog signal value to a digital signal value. A central processing unit (hereinafter referred to as a "CPU") 5b, a storage means 5c comprising a ROM and a RAM for storing various operation programs executed by the CPU 5b, various maps and operation results to be described later, and the fuel injection valve. And an output circuit 5d for supplying a drive signal to the ignition plug 13.

The CPU 5b determines various engine operating states such as a feedback control operating area and an open loop control operating area according to the oxygen concentration in the exhaust gas based on the various engine parameter signals described above, and sets the engine operating state. Accordingly, the fuel injection time TOUT of the fuel injection valve 6 is calculated in synchronization with the TDC signal pulse based on Expression (1) in the case of the basic mode and based on Expression (2) in the case of the start mode. Storage means 5c (RA
M).

TOUT = TiM × KCMDM × KLAF × K1 + K2 (1) TOUT = TiCR × K3 + K4 (2) Here, TiM is a basic fuel injection time in the basic mode, specifically, the engine speed NE and the inside of the intake pipe. This is a basic fuel injection time set according to the absolute pressure PBA.
The storage means 5c stores the TiM map for determining the iM value.
(ROM).

TiCR is a basic fuel injection time in the start mode, and is set in accordance with the engine speed NE and the intake pipe absolute pressure PBA similarly to the TiM value, and a TiCR map for determining the TiCR value is stored. Means 5c (RO
M).

KCMDM is a corrected target air-fuel ratio coefficient, which is an air-fuel ratio correction value set based on a target air-fuel ratio coefficient KCMD calculated based on the operating state of the engine and an output value of the O2 sensor 17 as described later. It is set according to ΔKCMD.

KLAF is an air-fuel ratio correction coefficient, which is set so that the air-fuel ratio detected by the LAF sensor 16 matches the target air-fuel ratio during the air-fuel ratio feedback control, and depends on the engine operating state during the open-loop control. Is set to a predetermined value.

K1, K2, K3, and K4 are correction coefficients and correction variables calculated in accordance with various engine parameter signals, and include various parameters such as fuel consumption characteristics and acceleration characteristics according to the operating state of the engine for each cylinder. The predetermined value is set so as to optimize the characteristics.

FIG. 2 is a diagram showing the configuration of the LAF sensor 16.
3, as shown in FIG. 3, comprises a substrate 20 of a substantially rectangular parallelepiped solid electrolyte material having oxygen ion conductivity (for example, ZrO ₂ (zirconium dioxide). The substrate 20 includes first and second substrates. Oxygen ion conductive solid electrolyte walls 21 and 2
2 are formed in parallel with each other,
A first gas diffusion chamber (diffusion limited area) 23 is formed between the walls 21 and 22 (in the vertical direction in the figure).

The gas diffusion chamber 23 communicates with the inside of the exhaust pipe through an introduction hole 24, and exhaust gas is introduced through the introduction hole 24. A gas reference chamber 26 is formed between the first wall portion 21 and an outer wall portion 25 formed on the wall portion 21 side, so that air (reference gas) is introduced. .

Electrode pairs are provided on the inner and outer wall surfaces of the first and second solid electrolyte walls 21 and 22 so as to face each other with the electrodes interposed therebetween. That is, Pt is applied to both side surfaces of the first wall portion 21.
One electrode pair 27a, 27b made of (platinum) is provided so as to face each other, and a battery element (sensing cell) is provided.
The other pair of electrodes 29a and 29b are similarly provided on both side surfaces of the second wall to form an oxygen pump element (pumping cell) 30.

On the other hand, the outer wall 25 is provided with a heater (heating element) 31 for heating the battery element 28 and the oxygen pump element 30 to promote their activation.

As shown in FIG. 2, the inner electrodes 27b and 29b of the electrodes for the detecting element, that is, the electrodes on the gas diffusion chamber 23 side are connected in common, and the inversion of the operational amplifier circuit 41 through the line l. Connected to input terminal.

On the other hand, the outer electrode 27a of the battery element 28 is connected to the inverting input terminal of the differential amplifier circuit 42. The differential amplifier circuit 42 includes a reference voltage source 43 connected to its non-inverting input terminal and an electrode pair 27a,
The voltage corresponding to the difference between the voltage between the electrodes 27b (in this case, the voltage obtained by adding the voltage on the line 1) and the reference voltage on the side of the reference voltage source 43 is applied to the oxygen pump element 30.
This constitutes means for applying voltage between the pair of electrodes 29a and 29b on the side.

The reference voltage VSO of the reference voltage source 43 is
In the present embodiment, when the supply air-fuel ratio is equal to the stoichiometric mixture ratio, a voltage (for example, 0.45 V) generated in the battery element 28 and a later-described reference voltage applied to a non-inverting input terminal of the operational amplifier circuit 41 are described. It is set to the sum voltage.

The output terminal of the differential amplifier circuit 42 is connected to the outer electrode 29a of the oxygen pump element 30. The voltage applied to the outer electrode 29a of the oxygen pump element 30 becomes positive or negative depending on whether the supply air-fuel ratio is lean or rich with respect to the stoichiometric mixture ratio, as described later. As a result, the applied voltage value changes, and the direction (positive or negative) of the pump current IP flowing through the oxygen pump element 30 and the line l to the pump current detecting resistor 46 described later also changes accordingly.

A reference voltage source 45 is connected to the non-inverting input terminal of the operational amplifier circuit 41, and between the output terminal of the operational amplifier circuit 41 and the line 1, that is, the operational amplifier circuit 4
A current detection resistor 46 for detecting a pump current is connected between the inverting input terminal 1 and the inverting input terminal. Therefore, the resistor 46 is inserted in the negative feedback path of the operational amplifier circuit 41.

In this configuration, when the pump current does not flow through the line 1, that is, when IP = 0, the voltage IPV at the output terminal of the operational amplifier circuit 41 (that is, one end of the pump current detecting resistor 46). Voltage) is the reference voltage source 45
And if IP = 0, the voltage VAFCENT at the inverting input end, that is, the potential on the line 1 and the other end of the current detection resistor 46 Can be made equal to the reference voltage source voltage value VREF.

Moreover, not only this, but also the pump current IP
Flows in the lean region and the rich region according to the supply air-fuel ratio, as described later, even if the voltage at the inverting input terminal of the operational amplifier circuit 41, that is, the current detection resistor 46 connected to the line l The voltage at the terminal can be substantially equal to the voltage at the non-inverting input terminal, that is, the reference voltage source voltage value VREF, regardless of the change in the pump current IP.

As described above, the voltage on the line l, that is, the other end voltage VAFCENT of the current detection resistor 46 is always substantially equal to VRE regardless of the presence or absence of the pump current and its change.
Thus, the voltage VAF at one end of the current detection resistor 46 connected to the output terminal side of the operational amplifier circuit 41 depends on the direction (positive or negative) of the pump current IP and the voltage VAF. Since the voltage VAFCENT changes according to the magnitude, the voltage VAFCENT becomes a center value (center voltage) when the current flowing through the oxygen pump element 30 is detected and the air-fuel ratio is calculated based on the detected current value. And the current detection resistor 4
6, the output voltage VAF of the operational amplifier 41,
The voltage VAFCENT of the line 1 is supplied to an input circuit 5a of the ECU 5.

Therefore, the input circuit 5a is supplied with VAFCENT indicating the center voltage value and VAF when calculating the air-fuel ratio based on the pump current IP.

The detection of the oxygen concentration by the LAF sensor 16 is performed as follows on the lean side and the rich side of the air-fuel ratio.

That is, when the exhaust gas is introduced into the gas diffusion chamber 23 through the introduction hole 24 with the operation of the engine, the inside of the gas diffusion chamber 23 and the gas reference chamber 2 into which the atmosphere is introduced are set.
6 and a difference in oxygen concentration occurs. A voltage (sensor voltage) is generated between the electrodes 27a and 27b of the battery element 28 according to the oxygen concentration difference, and a voltage obtained by adding the voltage between the electrodes 27a and 27b and the line 1 voltage VAFCENT is differentially amplified. It is supplied to the inverting input of the circuit 42. When the supply air-fuel ratio is on the lean side, the electrodes 27a,
27b, while the voltage V on line 1
Since AFCENT is maintained at VREF, the sum of the voltage between the electrodes 27a and 27b and the voltage VAFCENT becomes lower than the reference voltage VSO. As a result, the output level of the differential amplifier circuit 42 becomes a positive level, and this positive level voltage is applied to the oxygen pump element 30. When the oxygen pump element 30 is in an active state by the application of the positive level voltage, oxygen in the gas diffusion chamber 23 is ionized and released through the electrode 29b, the second wall 22, and the electrode 29a. Thus, the pump current IP is pumped out of the LAF sensor 16, and the pump current IP flows from the electrode 29 a toward the electrode 29 b, and flows through the current detection resistor 46 through the line 1. In this case, the pump current IP flows through the resistor 46 in a direction from the line 1 to the output end of the operational amplifier 41.

On the other hand, when the supply air-fuel ratio is on the rich side, the added voltage of the voltage between the electrodes 27a and 27b of the battery element 28 and the voltage VAFCENT on the line 1 is equal to the reference voltage VS.
When O is larger than O, the output level of the differential amplifier circuit 42 becomes a negative level, and external oxygen is supplied to the gas diffusion chamber 23 through the oxygen pump element 30 by the reverse operation.
And the pump current IP is
b flows toward the current 29a. In this case, the direction of the pump current IP on the line 1 is reversed, and the pump current IP flows through the current detecting resistor 46 in a direction opposite to the direction on the lean side described above.

When the supply air-fuel ratio is equal to the stoichiometric mixture ratio, the added voltage of the voltage between the electrodes 27a and 27b of the battery element 28 and the voltage VAFCENT becomes equal to the reference voltage VSO. No pumping or pumping is performed, and therefore no pump current flows (ie, in this case, the pump current value IP is IP = 0).

As described above, the pumping and pumping of oxygen are performed so that the oxygen concentration in the gas diffusion chamber 23 becomes constant, and the pump current flows. Therefore, the pump current value IP is determined on the lean side of the supplied air-fuel ratio. And on the rich side, they are proportional to the oxygen concentration of the exhaust gas.

Next, the air-fuel ratio feedback control method of the present invention executed by the CPU 5b will be described in detail.

FIG. 4 is a flowchart showing a main routine of the air-fuel ratio feedback control.

First, in step S1, the LAF sensor 16
Read the output value from. Next, it is determined whether or not the engine is in the start mode (step S2). Here, whether or not the engine is in the start mode is determined, for example, based on whether or not a starter switch (not shown) of the engine is turned on and whether or not the engine speed is equal to or lower than a predetermined start speed (cranking speed).

Then, the answer to step S2 is affirmative (YE
In the case of S), that is, in the start mode, the engine is at a low water temperature. The KTWLAF map which is a function of the engine cooling water temperature TW and the absolute pressure PBA in the intake pipe is searched to obtain a target air-fuel ratio coefficient at the low water temperature. KTWLAF is calculated (step S3), and the KTWLAF value is set to the target air-fuel ratio coefficient KCMD (step S4). Next, the flag FLAFFB is set to "0" to stop the feedback control of the air-fuel ratio (step S5), and the air-fuel ratio correction coefficient K
LAF and its integral term (I term) KLAFI are set to 1.0 (step S6, step S7), and this program is terminated.

On the other hand, when the answer to step S2 is negative (NO), that is, in the basic mode, the corrected target air-fuel ratio coefficient KCMDM is calculated based on the flowchart of FIG. 5 described later (step S8), and then the flag FACT is set. It is determined whether or not the value is "1" to determine whether or not the LAF sensor 16 is activated (step S9). Where LA
The activation determination of the F sensor 16 is performed by a LAF sensor activation determination routine (not shown) that is processed in the background. For example, the output voltage VOU of the LAF sensor 16 is determined.
When the difference between T and the center voltage VCENT is smaller than a predetermined value (for example, 0.4 V), it is determined that "LAF sensor 16 has been activated".

Then, the answer to step S9 is negative (NO).
If the answer is affirmative (YES) in step S9, that is, if the activation of the LAF sensor 16 has been completed, the process proceeds to step S10, where LA
Equivalent ratio KACT of air-fuel ratio detected by F sensor 16
(14.7 / (A / F)) (hereinafter, “detected air-fuel ratio coefficient”)
Is calculated. Here, the detected air-fuel ratio coefficient KAC
In consideration of the fact that the exhaust pressure fluctuates due to fluctuations in the absolute pressure PBA in the intake pipe, the engine speed NE, and the atmospheric pressure PA, T
It is calculated to a value corrected according to these operation parameters, and specifically, it is calculated by executing a KACT calculation routine of FIG. 6 described later.

Next, at step S11, a feedback processing routine is executed, and this program is terminated. That is, when the predetermined feedback condition is not satisfied, the flag FLAFFB is set to “0” to prohibit the feedback control, while when the predetermined feedback condition is satisfied, the flag FLAFFB is set to “1” to correct the air-fuel ratio. The coefficient KLAF is calculated, the execution of the feedback control is commanded, and the present program ends.

FIG. 5 is a flowchart of the KCMDM calculation routine executed in step S8 (FIG. 4). This program is executed in synchronization with the generation of the TDC signal pulse.

First, it is determined whether or not the engine 1 is in fuel cut (fuel supply stopped) (step S21).
Whether or not the fuel is cut is determined by the engine speed N
The determination is made based on E and the valve opening degree θTH of the throttle valve 3 ', and specifically, is determined by executing a fuel cut determination routine (not shown).

If the answer to step S21 is negative (N
In the case of O), that is, in the basic mode, step S
Proceeding to 22, the target air-fuel ratio coefficient KCMD is calculated. The target air-fuel ratio coefficient KCMD is normally read from a KCMD map in which map values KCMD are given in a matrix in accordance with the engine speed NE and the intake pipe absolute pressure PBA. Is appropriately corrected at the time of high-load operation, specifically, KCMD
By executing a calculation routine (not shown), the value is set to a value suitable for these operating conditions.

On the other hand, if the answer to step S21 is affirmative (YE
S), the target air-fuel ratio coefficient KCMD is set to a predetermined value KCM.
DFC (for example, 1.0) (step S2
3), proceed to step S24.

Next, in step S24, O2 processing is performed. That is, as described later, the target air-fuel ratio coefficient KCMD based on the output value from the O2 sensor 17 under a predetermined condition.
Is corrected to calculate the corrected target air-fuel ratio coefficient KCMDM.

Then, in step S25, a limit check of the corrected target air-fuel ratio coefficient KCMDM is performed, the program is terminated, and the routine returns to the main routine (FIG. 4). That is, the magnitude relationship between the KCMDM value calculated in step S24 and the predetermined upper and lower limit values KCMDMH and KCMDML is compared. If the KCMDM value is larger than the upper limit value KCMDMH, the KCMDM value is set to the upper limit value KCMDMH, and the KCMDM value is set. Is smaller than the lower limit KCMDML, the KCMDM value is set to the lower limit KCMDML.

FIG. 6 is a flowchart of the KACT calculation routine executed in step S10 (FIG. 4). This program is executed in synchronization with the generation of the TDC signal pulse.

First, in step S31, the storage unit 5c
The exhaust pressure map (exhaust pressure) PBOUT corresponding to the engine speed NE and the intake pipe absolute pressure PBA is calculated by searching the exhaust pressure map stored in the engine. The exhaust pressure map is provided with a predetermined map value for each of a plurality of engine operation areas determined by the engine speed NE and the intake pipe absolute pressure PBA. Is read out or calculated by an interpolation method.

In the following step S32, the aforementioned step S
The calculation of the exhaust pressure correction coefficient KPEX1 corresponding to the exhaust pressure PBOUT calculated in 31 is performed by using K stored in the storage unit 5c.
This is performed by searching the PEX1 table. Specifically, as shown in FIG. 7, the KPEX1 table includes exhaust pressure table values PBOUT1 to PBOUT corresponding to high to low loads of the engine.
For OUT4, exhaust pressure correction coefficient values KPEX11 to KPEX
EX41 are given, and the exhaust pressure correction coefficient KP
EX1 is read out by searching the KPEX1 table or calculated by an interpolation method. The exhaust pressure correction coefficient KPEX1 is set to increase as the value of the exhaust pressure PBOUT decreases. Further, in step S33, the exhaust pressure correction coefficient KPEX2 corresponding to the atmospheric pressure PA detected by the PA sensor 18 is calculated by searching the KPEX2 table stored in the storage unit 5c. The KPEX2 table is specifically shown in FIG.
Atmospheric pressure table value P corresponding to high altitude to low altitude as shown in
For AEX1 to PAEX4, the exhaust pressure correction coefficient value KPE
X21 to KPEX24 are given, and the exhaust pressure correction coefficient KPEX2 is read out by searching the KPEX2 table or calculated by an interpolation method. Note that the exhaust pressure correction coefficient KPEX2 is equal to the atmospheric pressure PA.
Is set to be larger as is larger.

Then, in step S34, a final exhaust pressure correction coefficient KPEX is calculated by the equation (3).

KPEX = KPEX1 × KPEX2 (3) In the next step S35, the error of the center voltage VAFCENT output from the LAF sensor 16 is learned and corrected. This learning correction is specifically performed by V
This is performed by executing the AFCENT correction routine. In addition,
This program is executed in synchronization with the generation of the TDC signal pulse.

In FIG. 9, first, in step S41, the O2 sensor 17 provided on the downstream side of the catalyst device 15 is used.
Output value VRO2 is a predetermined low level LVRO2 (0.
3V) and a predetermined high level HVRO2
(0.6 V), that is, whether the conversion efficiency (purification efficiency) of the catalyst device 15 is within an output value range in which the conversion efficiency is maximum. When the output value VRO2 of the O2 sensor 17 falls within the range of LVRO2 to HVRO and the answer is affirmative (YES), the process proceeds to step S42, and the read center voltage VAFCENT is learned and corrected by the equation (4). .

VAFCENT = (X_CREFX × VAF + 1−X_CREFX) × VAFCENT (4) Here, X_CREFX in Expression (4) is a predetermined learning smoothing coefficient, and VAF is the current output voltage VAF of the LAF sensor 16. It is a read value, VAFCENT on the right side
Is the current read value of the center voltage VAFCENT. Also, (X_CREFX × VAF + 1 = X_CRE) on the right side
FX) corresponds to an error of the read value VAFCENT.

In the following step S43, the corrected VA
VAF shown in FIG. 10 for FCENT value limit check
This is executed by a CENT limit check routine, whereby the corrected VAFCENT value is set within a predetermined upper and lower limit value. Specifically, as shown in FIG. 10, the corrected VAFCENT value is equal to a predetermined lower limit value VAFCENT.
It is determined whether or not it is larger than L (step S431). If the answer is negative (NO), the VAFCENT value is set to a predetermined lower limit value VAFCENTL (step S43).
2) This routine ends. On the other hand, if the answer is affirmative (YE
In the case of S), the VAFCENT value is set to the predetermined upper limit value VAF.
It is determined whether it is smaller than CENTH (step S43).
3) If the answer is negative (NO), VAFCENT
The value is set to a predetermined upper limit value VAFCENTL (step S434), and this routine ends. When the answer to step S433 is affirmative (YES), VAFCEN
The T value is a predetermined upper limit value VAFCENTH and a predetermined lower limit value V
This is a case where the value is between AFCENTL, and the VAFCENT value calculated in step S42 of FIG. 9 is held, and this routine ends.

On the other hand, if the answer in step S41 is negative (N
If the answer is O), the previous VAFCENT value is set as the current value (step S44), and this routine ends, and the process returns to step S36 in FIG.

In step S36 in FIG. 6, the output voltage VAF from the LAF sensor 16 is corrected by the equation (5) based on the VAFCENT value learned and corrected as described above. VAF = (VAFAD + VAFC.I−VAFCENT) × KPEX (5) Here, VAFAD in Expression (5) is a read value of the current output voltage VAF, VAFC.I is an initial value of the VAF value, and VAFCENT. Is the center voltage VAFCENT learned and corrected in step S35.

According to the above equation (5), the current read value VAFAD of the output voltage VAF is corrected in accordance with the learned corrected center voltage VAFCENT.

In the subsequent step S37, the detected air-fuel ratio coefficient KACT corresponding to the VAF value corrected in step S36 is stored in the storage unit 5c.
Search is performed using the CT table. Specifically, the KACT table, as shown in FIG. 11, compares the detected air-fuel ratio coefficient KAC with the VAF table values VAF0 to VAF11.
T0 to KACT11 are provided, and are created based on the initial value VAFC.I. The detected air-fuel ratio coefficient KACT is read out by searching the KACT table, or is calculated by an interpolation method. Note that the detected air-fuel ratio coefficient KACT and the VAF value are set so as to be approximately proportional.

As described above, in the KACT calculation routine of the present embodiment, the error occurs in the center voltage VAFCENT of the LAF sensor 16 due to the variation or deterioration of the amplifier circuit connected to the LAF sensor 16 or the sensor itself. Is corrected for learning in step S35. Since the learning correction of the VAFCENT value is performed when the output value of the O2 sensor 7 is in the range where the conversion efficiency of the catalyst device 15 is maximized, the variation of the LAF sensor 16 is absorbed and the best correction of the catalyst device 15 is always performed. It is possible to control the air-fuel ratio to obtain the purification efficiency. Next, the ECU 5 uses the learned and corrected VAFCENT value.
The output voltage VAF of the LAF sensor 16 read into the K is corrected, and the corrected output voltage VAF is used to calculate the K value.
By searching the ACT table, the center voltage V
It is possible to obtain an accurate detected air-fuel ratio coefficient KACT in consideration of the error of AFCENT.

FIG. 12 is a flowchart showing the operation in step S24.
6 is a flowchart of an O2 processing routine executed in (FIG. 5), and this program is executed in synchronization with generation of a TDC signal pulse.

First, in a step S51, it is determined whether or not the flag FO2 is "1", and it is determined whether or not the O2 sensor 17 is activated. Whether or not the O2 sensor 17 has been activated is specifically determined by executing an O2 sensor activation determination routine shown in FIG. This O2 sensor activation determination routine is executed during background processing.

First, in step S71, when an ignition switch (not shown) is turned on, a predetermined value (for example, 2.
Activation determination timer tmO2 set to 56 sec)
Is determined to be "0". If the answer is negative (NO), the O2 sensor 17 has not been activated yet, the flag FO2 is set to "0" (step S72), and the O2 sensor forced activation timer tmO2A is set.
CT is set to a predetermined value T1 (for example, 2.56 seconds), and the timer tmO2ACT is started (step S1).
73) Terminate this program.

On the other hand, if the answer to step S71 is affirmative (YE
In the case of S), it is determined whether or not the engine is in the start mode (step S74). When the answer is affirmative (YES), the forcible activation timer tmO2ACT is set to the predetermined value T1, and The timer tmO2ACT is started (step S73), and the program ends.

On the other hand, if the answer to step S74 is negative (NO)
If, the process proceeds to step S75, and it is determined whether or not the forcible activation timer tmO2ACT has become "0" (step S75). If the answer is negative (NO), the program ends, while the answer is affirmative (Y
In the case of (ES), it is determined that the O2 sensor 17 has been activated, and the flag FO2 is set to "1" (step S76), and this program ends.

As a result of executing the O2 sensor activation determination routine in this manner, step S51 is performed.
When the answer to FIG. 12 is negative (NO), that is, when it is determined that the O2 sensor 17 has not been activated yet, the process proceeds to step S52, and the timer tmRX is reset to a predetermined value T2 (for example, 0.25 sec). After setting, the flag FVREF
Is determined to be "0", and it is determined whether or not an initial value VRREF of the target correction value VREF of the O2 sensor 17 (hereinafter, referred to as "initial correction value") is already set (step S53).

Then, in the first loop, step S5
3 is affirmative (YES), the process proceeds to step S54, and the VRR stored in the storage unit 5c (ROM) is read.
The initial correction value VRREF is calculated by searching an EF table.

The VRREF table specifically contains P
A table value VRREF is given in steps to the atmospheric pressure PA detected by the A sensor 18, and the correction initial value VRREF is read out by searching the VRREF table or calculated by an interpolation method. Note that the correction initial value VRREF is set to a larger value as the value of the atmospheric pressure PA is larger.

Next, in step S55, the integral term (I term) VREFI (n-
1) is set to the correction initial value VRREF, the program is terminated, and the process returns to the main routine (FIG. 4). That is, initialization is performed for the target correction value VREFI (n-1) of the I term, and the process returns to the main routine (FIG. 4). still,
When step S53 is executed after the next loop,
Since the correction initial value of the target correction value has already been set as described above, the answer is negative (NO), and step S3 is performed.
This program ends without executing steps 4 and 35.

If the answer in step S51 is affirmative (Y
ES), it is determined that the O2 sensor 17 has been activated, and the flow advances to step S56 to execute the timer tmRX.
Is determined to be “0”. If the answer is negative (NO), the process proceeds to step S33, while if the answer in step S56 is affirmative (YES), the O2 sensor 1
7, the process proceeds to step S57, and determines whether the target air-fuel ratio coefficient KCMD set in step S22 or S23 (FIG. 5) is larger than a predetermined lower limit KCMDZL (for example, 0.98). Is determined. And
If the answer is negative (NO), it means that the air-fuel ratio of the air-fuel mixture has been set to the lean burn state, and this program is ended. On the other hand, if the answer is affirmative (YES), the flow proceeds to step S58. It is determined whether or not the target air-fuel ratio coefficient KCMD is smaller than a predetermined upper limit value KCMDZH (for example, 1.13). If the answer is negative (NO), it means that the air-fuel ratio of the air-fuel mixture is set to fuel rich, and this program is terminated while the answer is affirmative (Y
ES), the air-fuel ratio of the mixture is the stoichiometric air-fuel ratio (A / F
= 14.7), which should be set in step S59.
To determine whether or not the engine is in the fuel cut mode. If the answer is affirmative (YES), the program ends and returns to the main routine (FIG. 4).
When the answer is negative (NO), it is determined whether or not the fuel cut was performed in the previous loop (step S60). If the answer is affirmative (YES), the counter NAFC is set to a predetermined value N1 (for example, 4) (step S61), and the counter value N1 of the counter NAFC is decremented by "1" (step S61). S62) This program ends.

On the other hand, if the answer to step S60 is negative (NO)
When it becomes, the process proceeds to step S63, and the counter NAF
It is determined whether or not C is "0". When the answer is negative (NO), the count value of the counter NAFC is decremented by "1" (step S62), and the program is terminated. On the other hand, when the answer is affirmative (YES), the fuel cut is performed. It is determined that stable fuel supply is being performed after exiting the state.
After executing the feedback processing, the program ends, and the process returns to the main routine (FIG. 4).

FIG. 14 is a flowchart showing the operation in step S64.
13 is a flowchart of an O2 feedback processing routine executed in (FIG. 12), and this program is executed in synchronization with generation of a TDC signal pulse.

First, in step S81, the thinning variable N
It is determined whether or not the IVR is “0”. This thinning variable NI
VR is a variable that is decremented each time a TDC signal pulse is generated by the number of thinned TDC NI set in accordance with the engine operating state, as described later. Since VR is initially "0", the answer of step S81 is The result is affirmative (YES), and the process proceeds to step S82.

If the answer to step S81 is negative (NO) in the subsequent loop, the process proceeds to step S83, and the value obtained by subtracting the number of thinned TDCs NI (for example, 1) from the thinned variable NIVR is used as a new thinned variable. This is set to NIVR and this routine ends.

At step S82, the KVP map and the KV
Search I map, KVD map, NIVR map and
2 Change rate of feedback control, that is, the proportional term (P
Term) coefficient KVP, integral term (I term) coefficient KVI, derivative term (D term) coefficient KVD, and the thinning number NIVR are calculated. In the KVP map, the KVI map, the KVD map, and the NIVR map, predetermined map values are given for each of a plurality of engine operating regions determined by the engine speed NER and the intake pipe absolute pressure PBA. Is read out or calculated by an interpolation method. still,
The KVP map, the KVI map, the KVD map and the NIVR map are for a steady operation state and when the operation mode is changed.
The dedicated map is stored in advance in the storage unit 5c (RO) so that the optimum value is set according to each operation state of the engine such as the deceleration operation state.
M).

Next, in step S83, the thinning variable NIV
After setting R to the NIVR value calculated in step S82, the process proceeds to step S84, and the process proceeds to step S54.
A deviation ΔV (n) between the correction initial value VRREF calculated in (FIG. 11) and the output voltage VO2 of the O2 sensor 17 in the current loop is calculated.

Next, in step S85, equations (6) to (6)
Based on (9), the target correction values VREFP (n), VREFI (n), V
After calculating REFD (n), based on equation (9), these correction terms are added to calculate a target correction value VREF (n) in O2 feedback.

VREFP (n) = ΔV (n) × KVP (6) VREFI (n) = VREF + ΔV (n) × KVI (7) VREFD (n) = (ΔV (n) −ΔV (n−1)) ) × KVD (8) VREF (n) = VREFP (n) + VREFI (n) + VREFD (n) (9) Next, in step S86, a limit check of VREF (n) is performed. This limit check is specifically executed according to the flowchart shown in FIG. This program is executed in synchronization with the generation of the TDC signal pulse. First, in step S91, the target correction value VREF (n) is set to a predetermined lower limit VREFL (for example,
0.2 V) is determined. If the answer is negative (NO), the target correction value VREF (n)
And the I-term target correction value VREFI (n) is set to the predetermined lower limit value VREFL, respectively (steps S92 and S93).
Exit this program.

On the other hand, if the answer in step S91 is affirmative (YE
In the case of S), it is determined whether or not the target correction value VREF (n) is smaller than a predetermined upper limit value VREFH (for example, 0.8 V) (step S94). And the answer is affirmative (Y
ES) is the case where the target correction value VREF (n) is between the predetermined upper limit value VREFH and the predetermined lower limit value VREFL, and VREF calculated in step S98.
(N) While ending this program while retaining the value,
If the answer to step S94 is negative (NO), the target correction value VREF (n) and the I-term target correction value VREFI (n)
Is set to the predetermined upper limit value VREFH (step S9).
5, S96) This program ends.

After the limit check of VREF (n) is completed, the process proceeds to step S87 (FIG. 14),
An air-fuel ratio correction value ΔKCMD is calculated.

The air-fuel ratio correction value ΔKCMD is, specifically, Δ
It is calculated by searching the KCMD table. That is,
In the ΔKCMD table, a table value ΔKCMD is given to the target correction value VREF, and the air-fuel ratio correction value ΔKCMD is read out by searching the ΔKCMD table or calculated by an interpolation method. still,
The ΔKCMD value becomes larger as VREF (n) has a larger value.
Usually set to a large value. Further, since the limit check is performed in step S86 for the VREF value, the ΔKCMD value is also set to a value within the predetermined upper and lower limit values.

Next, at step S88, the corrected target air-fuel ratio coefficient KCMDM (= theoretical air-fuel ratio) is calculated by adding the air-fuel ratio correction value ΔKCMD to the target air-fuel ratio coefficient KCMD calculated at step S22 (FIG. 3). , End this program.

As described above, in the present embodiment, learning correction of the center voltage VAFCENT as the output value of the LAF sensor 16 is performed when the output value of the O2 sensor 7 is in the range where the conversion efficiency of the catalyst device 15 is maximized. Since the accurate detected air-fuel ratio coefficient KACT is determined in consideration of the error of the center voltage VAFCENT of the LAF sensor 16 thus learned and corrected, the air-fuel ratio of the air-fuel mixture supplied to the engine is set as the target air-fuel ratio. Therefore, it is possible to reliably control the stoichiometric air-fuel ratio at which the maximum purification efficiency is obtained.

[0102]

As described above, according to the present invention, when the output value of the second exhaust gas concentration sensor is in the range in which the conversion efficiency of the catalyst device is at a maximum, the first exhaust gas concentration sensor can perform the first operation. The output value correction means for correcting the output value of the exhaust gas concentration sensor, preferably the center voltage, is provided, so that the variation in the output characteristics of the first exhaust gas concentration sensor can be corrected and the air-fuel ratio control can always be performed with high accuracy. Thus, the exhaust gas characteristics can be improved.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram showing an embodiment of an air-fuel ratio control device for an internal combustion engine according to the present invention.

FIG. 2 is a diagram illustrating a configuration example of a LAF sensor.

FIG. 3 is a perspective view showing a main body of the LAF sensor.

FIG. 4 is a flowchart illustrating a main routine of air-fuel ratio feedback control.

FIG. 5 is a flowchart showing a KCMDM calculation routine.

FIG. 6 is a flowchart illustrating a KACT calculation routine.

FIG. 7 is a diagram showing a KPEX1 table.

FIG. 8 is a diagram showing a KPEX2 table.

FIG. 9 is a flowchart illustrating a VAFCENT correction routine.

FIG. 10 is a flowchart of a limit check routine for a corrected VAFCENT value.

FIG. 11 is a diagram showing a KACT table.

FIG. 12 is a flowchart showing an O2 processing routine.

FIG. 13 is a flowchart illustrating an O2 sensor activation determination routine.

FIG. 14 is a flowchart showing an O2 feedback control routine.

FIG. 15 is a flowchart of a VREF (n) limit check routine.

[Explanation of symbols]

 Reference Signs List 1 engine 5 ECU 6 fuel injection valve 14 exhaust pipe 15 catalyst device 16 LAF sensor 17 O2 sensor

Claims (1)

(57) [Claims] 1. A catalyst device disposed in an exhaust passage of an internal combustion engine for purifying harmful components in exhaust gas, and an output characteristic disposed in an exhaust passage upstream of the catalyst device and substantially proportional to an exhaust gas concentration. A first exhaust gas concentration sensor having an operating condition detecting means for detecting an operating condition of the engine, and a target for calculating a target air-fuel ratio of an air-fuel mixture supplied to the engine based on a detection result of the operating condition detecting device. An air-fuel ratio calculating means, a second exhaust gas concentration sensor disposed in the exhaust passage downstream of the catalyst device and having an output signal inverted near a target air-fuel ratio, and an output value of the second exhaust gas concentration sensor. And a target air-fuel ratio corrector that corrects the target air-fuel ratio calculated by the target air-fuel ratio calculator based on the target air-fuel ratio and the output value of the first exhaust gas concentration sensor. Supplied to the engine An air-fuel ratio control device for an internal combustion engine that controls an air-fuel ratio of an air-fuel mixture to be supplied.
When in range of conversion efficiency is maximized, the internal combustion engine, characterized in that an output value correction means for correcting the center value corresponding to the target air-fuel ratio which is an output value of said first exhaust gas component concentration sensor Air-fuel ratio control device.