JPH05321653A - Exhaust emission control device of internal combustion engine - Google Patents

Abstract

PURPOSE:To obtain an exhaust emission control device of an internal combustion engine which is capable of improving the purifying characteristics and to suppress the deterioration of a catalyst device by effectively removing the harmful contens such as NOx, HC and CO. CONSTITUTION:The target air fuel ratio of a first wide-range oxygen concentration sensor (LAFF sensor) 17 is set slightly richer based on the detected value of an 02 sensor 18, and NOx is fully removed by using a first and a second catalyst devices 15, 19. The secondary air is supplied from a secondary air supplying system 22 to a third catalyst device 20, and HC and CO are sufficiently removed by the third catalyst device. In addition, the feedback control of the secondary air flow rate to be supplied from the secondary air supplying system 22 to a secondary air introducing port 21 is executed based on the output value of a second wide-range oxygen concentration sensor (LAFR sensor) 50.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an exhaust gas purifying device for an internal combustion engine, and an exhaust gas purifying device for an internal combustion engine, which removes harmful components in the exhaust gas by a catalyst device arranged in an exhaust system passage of the internal combustion engine. ..

[0002]

2. Description of the Related Art Conventionally, a three-way catalyst, which is a catalytic device, is disposed in an exhaust system of an internal combustion engine to remove harmful components such as NOx, HC and CO discharged from the engine. Is widely known.

As an exhaust gas purifying apparatus for an internal combustion engine of this type, oxygen concentration sensors (O2 sensors) are arranged on the upstream side and the downstream side of the catalyst device, respectively, and the catalyst is detected based on the output values of these two O2 sensors. A technique has been already proposed in which the air-fuel ratio is feedback-controlled so that the air-fuel ratio of the air-fuel mixture in the device accurately becomes the stoichiometric air-fuel ratio, and the performance of the three-way catalyst, which is a catalyst device, is maximized. -2244
No. 26) (hereinafter referred to as "first conventional example").

When removing the harmful components, the air-fuel ratio of the air-fuel mixture in the catalyst device is set to a rich atmosphere for removing NOx, while the air-fuel ratio of the air-fuel mixture in the catalyst device for removing HC and CO is set to a rich atmosphere. It is known that these harmful components can be effectively removed by setting the air-fuel ratio to a lean atmosphere, and the first and second catalyst devices are arranged in series in the exhaust system of the engine and the first catalyst is also used. A technique has already been proposed in which secondary air is introduced between the device and the second catalyst device (for example, Japanese Patent Publication No. 57-5).
No. 1525) (hereinafter referred to as "second conventional example").

In the second conventional example, the air-fuel ratio in the first catalytic device is detected by an oxygen sensor (O2 sensor), and the secondary air is supplied to the second catalytic device to produce the second catalytic converter. It is possible to effectively remove NOx by the first catalyst device and HC and CO by the second catalyst device by using the catalyst device as an oxidation catalyst.

[0006]

However, in the first conventional example described above, the performance of the three-way catalyst is maximized by feedback-controlling the air-fuel ratio of the air-fuel mixture in the catalyst device so that it accurately becomes the stoichiometric air-fuel ratio. Although it is utilized to the limit, as described above, the oxidizing component in the harmful components, especially HC, is better in purification rate when the air-fuel ratio is made leaner, and the air-fuel ratio of the catalyst device may be the theoretical air-fuel ratio. However, there was a problem that CO was not sufficiently purified.

Further, in the second conventional example, since the air-fuel ratio of the air-fuel mixture is detected by the O2 sensor, only a rich signal or a lean signal with respect to the stoichiometric air-fuel ratio can be detected and the catalyst showing the purification characteristic of the catalyst. The air-fuel ratio on the downstream side of the device is not used for feedback control, and even if the average value of the target air-fuel ratio is in a slightly rich state (air-fuel ratio (A / F) = 14.48).
.About.14.63), there is a problem that the actual fluctuation range of the air-fuel ratio is large and it is difficult for the first catalyst device to sufficiently remove NOx.

Further, in the second conventional example, when an excessive amount of secondary air is supplied to the second catalytic device, deterioration of the catalytic device due to an oxidizing atmosphere is promoted, resulting in poor durability. There was a point.

The present invention has been made in view of the above problems, and effectively removes harmful components such as NOx, HC, and CO to improve purification characteristics and suppress deterioration of the catalyst device. It is an object of the present invention to provide an exhaust gas purifying apparatus for an internal combustion engine that can achieve the above.

[0010]

In order to achieve the above object, an exhaust gas purifying apparatus for an internal combustion engine according to the present invention is arranged in an exhaust passage of an internal combustion engine to purify harmful components in exhaust gas. A catalyst device, a first exhaust gas concentration sensor having an output characteristic that is disposed in the exhaust passage upstream of the first catalyst device and is approximately proportional to the exhaust gas concentration, and at least the engine speed and the load state of the engine. An operating state detecting means for detecting an operating state of the engine, a first target air-fuel ratio calculating means for calculating a target air-fuel ratio based on a detection result of the operating state detecting means, and the downstream side of the catalyst device. A second exhaust gas concentration sensor in which an output signal is inverted in the vicinity of the target air-fuel ratio arranged in the exhaust passage, and the target air-fuel ratio is set to be slightly higher than the theoretical air-fuel ratio based on the output value of the second exhaust gas concentration sensor. rich And a control means for feedback-controlling the air-fuel ratio of the air-fuel mixture detected by the first exhaust gas concentration sensor to the target air-fuel ratio set by the enrichment means, and Second
A second catalyst device is arranged in the exhaust passage downstream of the second exhaust gas concentration sensor, and a third exhaust gas concentration sensor is arranged downstream of the second catalyst device; Secondary air supply means provided in the exhaust passage on the upstream side of the catalyst device and on the downstream side of the second exhaust gas concentration sensor;
An air flow rate control means for feedback controlling the flow rate of the secondary air from the secondary air supply means is provided based on the output value of the third exhaust gas concentration sensor.

It is also preferable to provide a third catalyst device in the exhaust passage between the second air-fuel ratio sensor and the secondary air supply means.

Also, the air flow rate control means is the third
The air-fuel ratio of the air-fuel mixture in the second catalyst device is controlled to a leaner state than the stoichiometric air-fuel ratio based on the output value of the exhaust gas concentration sensor.

Further, according to the present invention, there is provided valve timing switching means capable of switching the valve timing of the intake and exhaust valves of the engine to at least a low speed valve timing suitable for a low rotation range and a high speed valve timing suitable for a high rotation range. At the same time, the secondary air supply means calculates the basic air flow rate according to the setting state of the valve timing by the valve timing switching means and the detection result of the operating state detection means, and the third exhaust gas. Based on a correction coefficient calculation means for calculating an air-fuel ratio correction coefficient based on the output value of the concentration sensor, at least the basic air flow rate calculated by the air flow rate calculation means, and the air-fuel ratio correction coefficient calculated by the correction coefficient calculation means. And an air flow rate determining means for determining a secondary air flow rate.

[0014]

According to the above construction, the secondary air flow rate is determined based on the output value of the third exhaust gas concentration sensor so that the air-fuel ratio of the air-fuel mixture in the second catalyst device is in a slightly leaner state than the stoichiometric air-fuel ratio. It becomes possible to perform feedback control of the quantity.

The secondary air flow rate is determined based on at least the basic secondary air flow rate and the air-fuel ratio correction coefficient.

[0016]

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 system for an internal combustion engine according to the present invention.

In the figure, reference numeral 1 denotes a DOHC in-line 4-cylinder internal combustion engine (hereinafter simply referred to as "engine") in which each cylinder is provided with an intake valve and an exhaust valve (not shown). In the engine 1, the valve timing of the intake valve is in two stages: a high speed valve timing (high speed V / T) suitable for the high speed rotation range of the engine and a low speed valve timing (low speed V / T) suitable for the low speed rotation range. It is configured to be switchable.

A throttle body 3 is provided in the middle of an intake pipe 2 of the engine 1, and a throttle valve 3'is provided inside thereof.
Are arranged. Further, a throttle valve opening degree (θTH) sensor 4 is connected to the throttle valve 3 ', and an electric signal corresponding to the opening degree of the throttle valve 3'is output to output an electronic control unit (hereinafter referred to as "ECU") 5 Supply to.

The fuel injection valve 6 is provided for each cylinder in the middle of the intake pipe 2 between the engine 1 and the throttle valve 3 ', is connected to a fuel pump (not shown), and is electrically connected to the ECU 5. The valve opening time of fuel injection is controlled by a signal from the ECU 5.

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 the tip of the branch pipe 7. The PBA sensor 8 is electrically connected to the ECU 5, and the intake pipe 2
The absolute pressure PBA therein is converted into an electric signal by the PBA sensor 8 and supplied to the ECU 5.

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

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

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

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

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

A first catalyst device (three-way catalyst) 15 is provided midway in the exhaust pipe 14 of the engine 1, and the first catalyst device 15 causes HC, CO, and NO in the exhaust gas to be exhausted.
Purification of harmful components such as x is performed.

In this embodiment, the air-fuel ratio of the air-fuel mixture in the first catalyst device 15 is made to be rich fuel,
Purification of NOx is mainly performed.

Further, a first catalyst temperature (TCT) sensor 16 composed of a thermistor or the like is provided on the peripheral wall of the first catalyst device 15.
The catalyst bed temperature TCT detected by the TCT sensor 16 is converted into an electric signal and supplied to the ECU 5.

A first wide-range oxygen concentration sensor (hereinafter referred to as "LAFF sensor") 17 and an oxygen concentration sensor (hereinafter referred to as "LAFF sensor") are provided in the exhaust pipe 14 upstream and downstream of the first catalyst device 15, respectively. , "O2 sensor" 18 is provided.

The LAFF sensor 17 includes a pair of upper and lower battery elements and an oxygen pump element which are zirconia solid electrolyte (ZrO 2).
₂ ) is attached to a predetermined position of a sensor element such as, and the sensor element is electrically connected to an amplifier circuit. Then, the LAFF sensor 17 outputs an electric signal substantially proportional to the oxygen concentration in the exhaust gas passing through the inside of the sensor element, and supplies the electric signal to the ECU 5.

The sensor element of the O2 sensor 17 is the same as that of the LAFF sensor 17, and the zirconia solid electrolyte (Zr
O ₂ ), the electromotive force of which is abruptly changed before and after the stoichiometric air-fuel ratio, and the output signal is inverted from the lean signal to the rich signal or from the rich signal to the lean signal at the stoichiometric air-fuel ratio. That is, the O2 sensor 18
Output signal becomes high level on the exhaust gas rich side and becomes low level on the lean side, and the output signal is supplied to the ECU 5.

Further, a third catalyst device 19 and a second catalyst device 20 are arranged in series in the exhaust pipe 14 downstream of the O2 sensor 18.

The third catalyst device 19 has a function as a three-way catalyst like the first catalyst device 15, and in the present embodiment, the air-fuel ratio of the air-fuel mixture in the third catalyst device 19 is increased. As in the case of the first catalyst device 15, the purification of NOx is mainly performed by making the fuel rich in the fuel.

The second catalytic device 20 has a function as an oxidation catalyst, and the first secondary air provided between the third catalytic device 19 and the second catalytic device 20. Secondary air is supplied from the secondary air supply system (secondary air supply means) 22 through the introduction hole 21, and the air-fuel ratio of the air-fuel mixture is made lean to mainly purify HC and CO.

The first to third catalytic devices 15, 19, 2
As will be described later, the volume ratio of 0 is such that the removal of NOx by the first and third catalyst devices 15 and 19 and the second catalyst device 2 are performed.
In order to remove HC and CO most effectively at 0, the first and third catalytic devices 15, 15
The ratio of 19 to the second catalyst device 20 as an oxidation catalyst is set to 1.8: 1 to 2.2: 1.

A second catalyst temperature (TCO) sensor 23 including a thermistor or the like is provided on the peripheral wall of the second catalyst device 20.
The catalyst bed temperature TCO detected by the TCO sensor 23 is converted into an electric signal and supplied to the ECU 5.

On the downstream side of the second catalyst device 20,
Second wide area oxygen concentration sensor (hereinafter referred to as "LAFR sensor")
50) is provided. The LAFR sensor 50
Has a configuration similar to that of the LAFF sensor 16, outputs an electric signal substantially proportional to the oxygen concentration in the exhaust gas, and supplies the electric signal to the ECU 5.

An electromagnetic valve 24 for controlling the switching of the valve timing is connected to the output side of the ECU 5,
The opening / closing operation of the electromagnetic valve 24 is controlled by the ECU 5.
The solenoid valve 24 switches the hydraulic pressure of a switching mechanism (not shown) for switching the valve timing between high and low,
Valve timing is high speed V /
T and low speed V / T are switched. The hydraulic pressure of the switching mechanism is detected by a hydraulic pressure (POIL) sensor 25, and the detection signal is supplied to the ECU 5.

Further, the atmospheric pressure (PA) sensor 26 is arranged at a proper position of the engine 1, detects the atmospheric pressure PA, and supplies its electric signal to the ECU 5.

Therefore, the secondary air supply system 22 has an air pump 27 as an air supply source, an air chamber 28 for storing the air from the air pump 27, and a pressure adjusting valve 29.
And a secondary air control valve 30 and a two-position switching valve (hereinafter referred to as “switching valve”) 31. That is, an air pump 27 is connected to the air chamber 28 via an air supply path 33, and the air pump 27 is connected to the ECU 5
The air supply to the air chamber 28 is controlled by a signal from the ECU 5. A chamber pressure (PC) sensor 32 is attached to the air chamber 28, and the PC sensor 32 is electrically connected to the ECU 5, and the chamber pressure PC detected by the PC sensor 32 is converted into an electric signal. Is supplied to the ECU 5.

The pressure adjusting valve 29 and the secondary air control valve 30 are provided in the air supply passage 33 on the downstream side of the air chamber 28.
Have been installed respectively. The pressure adjusting valve 29 is electrically connected to the ECU 5 and adjusts the air pressure conveyed from the air chamber 28 by a signal from the ECU 5.

The secondary air control valve 30 has a wedge-shaped valve body 34 movably arranged in the up-and-down direction so that the air supply passage 33 can communicate with each other, and a casing having the valve body 34 therein. 35 and an electromagnetic valve 36 for driving the valve element 34 in the vertical direction
And a lift sensor (hereinafter referred to as “L sensor”) 37 connected to the valve element 34 via a valve shaft 38.
The solenoid valve 36 is electrically connected to the ECU 5,
The lift amount of the valve element 34 in the vertical direction is duty-controlled based on the electric signal from the CU 5. In addition, the L sensor 38
Detects the lift amount of the valve element 34 and supplies the detected lift amount to the ECU 5.

The switching valve 31 is arranged in the air supply passage 33 on the downstream side of the secondary air control valve 30, and also has the first and second air supply passages.
Are connected to the first secondary air introduction hole 21 and the second secondary air introduction hole 42, respectively. Further, the switching valve 31 is electrically connected to the ECU 5, and the air supply passage 3 is electrically connected based on an electric signal from the ECU 5.
3 and the first or second air supply branch paths 39, 40 can communicate with each other, and the first catalyst device 15 or the second catalyst device 2
0 is supplied with secondary air. That is, in the present embodiment, when the engine is started at a low temperature, the switching valve 31 brings the air supply path 33 and the first air supply branch path 39 into communication with each other, and the secondary air flows through the check valve 41 to the second side. By being supplied to the secondary air introduction hole 41, the exhaust gas from the engine 1 is mixed with air to make the air-fuel ratio of the air-fuel mixture lean, and the activation of the first catalyst device 15 is promoted. On the other hand, after the activation of the first catalyst device 15 is completed, the switching valve 31 is switched to bring the air supply path 33 and the second air supply branch path 40 into communication with each other, and the first valve via the check valve 43. The secondary air can be supplied to the secondary air introduction hole 21 and the amount of secondary air supplied to the second catalyst device 20 is controlled by the secondary air control valve 30.

Therefore, the ECU 5 forms an input signal waveform from the above-mentioned various sensors, corrects the voltage level to a predetermined level, and converts the analog signal value into a digital signal value. A central processing circuit (hereinafter referred to as "CPU") 5b, a storage means 5c including ROM and RAM for storing various calculation programs executed by the CPU 5b, various maps to be described later, and calculation results, and the fuel injection valve. 6, an output circuit 5 for supplying a drive signal to the spark plug 13, the air pump 27, the solenoid valve 36, etc.
and d.

Further, the CPU 5b determines various engine operating states such as a feedback control operating region and an open loop control operating region according to the oxygen concentration in the exhaust gas based on the various engine parameter signals described above, and Depending on the operating condition, the basic mode is based on the formula (1), and the starting mode is the formula (2).
Based on the fuel injection valve 6 in synchronization with the TDC signal pulse.
The fuel injection time TOUT is calculated and the result is stored in the storage means 5c (RAM).

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

TiCR is the 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 like the TiM value, and the TiCR map for determining the TiCR value is stored. Means 5c (RO
M).

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

KLAFF is an air-fuel ratio correction coefficient, which is set so that the air-fuel ratio detected by the LAFF sensor 17 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. It is set to a predetermined value.

K1, K2, K3, and K4 are correction coefficients and correction variables calculated according to various engine parameter signals, respectively, and various characteristics such as fuel consumption characteristics and acceleration characteristics according to the operating state of the engine for each cylinder. It is set to a predetermined value that optimizes the characteristics.

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

FIG. 2 is a flow chart showing the main routine of the air-fuel ratio feedback control.

First, in step S1, the LAFF sensor 1
Read the output value from 7. Next, it is determined whether the engine is in the starting mode (step S2). here,
Whether or not the engine is in the starting mode is determined by, for example, whether or not the starter switch of the engine (not shown) is on and the engine speed is equal to or lower than a predetermined starting speed (cranking speed).

Then, the answer in step S2 is affirmative (YE
S), that is, when the engine is at a low water temperature in the start mode, the KTWLAF map, which is a function of the engine cooling water temperature TW and the intake pipe absolute pressure PBA, is searched to retrieve the 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
LAFF and its integral term (I term) KLAFFI to 1.0
Is set (steps S6 and S7), and this program ends.

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. 4 described later (step S8), and then the flag FACT is set to " 1 "
It is determined whether or not the LAFF sensor 17 is activated (step S9). Where LAFF
The activation determination of the sensor 17 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 LAFF sensor 17 is determined.
When the difference between T and its center voltage VCENT is smaller than a predetermined value (for example, 0.4 V), it is determined that "LAFF sensor 17 has been activated".

Then, the answer to step S9 is negative (NO).
If so, the process proceeds to step S5, while if the answer to step S9 is affirmative (YES), that is, if the activation of the LAFF sensor 17 has been completed, the process proceeds to step S10.
Equivalent ratio KA of air-fuel ratio detected by AFF sensor 17
CT (14.7 / (A / F)) (hereinafter, referred to as “detected air-fuel ratio coefficient”) is calculated. Here, the detected air-fuel ratio coefficient K
ACT is the absolute pressure PBA in the intake pipe and the engine speed NE.
In view of the fact that the exhaust pressure fluctuates due to fluctuations in the atmospheric pressure PA and the atmospheric pressure PA, the values are calculated to values corrected according to these operating parameters, and specifically, a KACT calculation routine (not shown).
Is calculated by executing.

Next, in step S11, a feedback processing routine is executed and this program ends. That is, when the predetermined feedback condition is not satisfied, the flag FLAFFB is set to "0" to inhibit 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 KLAFF is calculated, the execution of the feedback control is instructed, and this program is ended.

FIG. 3 is a flow chart of the KLAFF calculation routine executed in the feedback process (step S11). This program is executed in synchronization with the generation of the TDC signal pulse.

First, at step S21, the air-fuel ratio deviation ΔKAFF between the target air-fuel ratio coefficient KCMD (n- ₁ ) in the previous loop and the detected air-fuel ratio coefficient KACT (n) in the current loop is calculated.

Next, at step S22, the air-fuel ratio correction coefficient K
Initialize LAFF and the like. That is, the initialization routine (not shown) initializes the air-fuel ratio correction pond number KLAFF according to the operating state of the engine.

Next, in step S23, the thinning T
It is determined whether or not the DC variable NITDC is "0". The thinned-out TDC variable NITDC is a variable for updating the air-fuel ratio correction coefficient KLAFF every time the TDC signal pulse is generated by the thinning-out number NI set according to the engine operating state, and the answer to step S23 is negative. (NO),
That is, when NITDC> 0, the NITDC value is set to "1".
Only (step S24) and the current value ΔKAF (n) of the air-fuel ratio deviation is changed to the previous value ΔKAF (n- ₁ ).
(Step S25) This program ends.

The answer to step S23 is affirmative (YE
S), that is, when NITDC = 0, step S26
The KLAFF value is updated by executing the following flow.

That is, the answer in step S23 is affirmative (Y
In the case of (ES), the KP map, KI map, KD map, and NI map are searched to change the air-fuel ratio feedback control, that is, the proportional term (P term) coefficient KP, the integral term (I term) coefficient KI, and the differential term KD. And the thinning-out number NI is calculated (step S26). KP map, KI map, KD map and NI map are engine speed N
E, a predetermined map value is given for each of a plurality of engine operating regions determined by the intake pipe absolute pressure PBA, and the like, and a map value corresponding to the operating state of the engine is read out or interpolated by searching these maps. Calculated by the method. The KP map, KI map, and KD map are pre-stored as dedicated maps so that optimum values can be set according to each operating state of the engine, such as a steady operating state, a change in operating mode, and a decelerating operating state. 5c (ROM)
Remembered in.

Next, in step S27, equations (2)-
Based on (4), P term KLAFFP, I term KLAFFI
And the D term KLAFF is calculated.

KLAFFP = ΔKAFF (n) × KP (2) KLAFFI = KLAFFI + ΔKAFF (n) × KI (3) KLAFDD = (ΔKAFF (n) −ΔKAFF (n− ₁ )) × KD (4) Step S28 Then, the limit check of the I term KLAFFI is performed. That is, the KLAFFI value and the predetermined upper and lower limit value LA
The magnitude relationship between FFIH and LAFFIL is compared. As a result, when the KLAFFI term is larger than the upper limit value LAFFIH, the upper limit value LAFFIIH is set and the lower limit value LAF is set.
When it is smaller than FIL, the lower limit value LAFIL is set.

In step S29, the P term KLAFFP,
The I term KLAFFI and the D term KLAFFD are respectively added to calculate the air-fuel ratio correction coefficient KLAFF, and then the present calculated value DKAF (n) of the air-fuel ratio deviation is changed to the previous value DKAF (n- ₁ ).
(Step S30), and further thinned-out TDC variable NI
TDC is set to the thinning number NI calculated in step S26 (step S31).

Next, in step S32, a limit check of KLAFF is performed, and this program ends.

FIG. 4 is a flow chart of the KCMDM calculation routine executed in step S8 (FIG. 2), and this program is executed in synchronization with the generation of the TDC signal pulse.

First, in step S41, it is determined whether or not the engine 1 is in the fuel cut (fuel supply is stopped).
Whether or not the fuel is being 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 is specifically determined by executing a fuel cut determination routine (not shown).

Then, the answer to step S41 is negative (N
In the case of O), the process proceeds to step S42 and the target air-fuel ratio coefficient KCMD is calculated. The target air-fuel ratio coefficient KCMD is usually a map value KCMD given in a matrix according to the engine speed NE and the intake pipe absolute pressure PBA.
It is read from the CMD map, but is corrected as appropriate when the vehicle starts, when the water temperature is low, or when the vehicle is operating under a predetermined high load. Specifically, these operating states are executed by executing a KCMD calculation routine (not shown). Is set to a value compatible with.

On the other hand, the answer in step S41 is affirmative (YE
In the case of S), the target air-fuel ratio coefficient KCMD is set to a predetermined value KCM.
Set to DFC (for example, 1.0) (step S4
3) and proceeds to step S44.

Next, in step S44, O2 processing is performed. That is, as will be described later, the target air-fuel ratio coefficient KCMD is based on the output value from the O2 sensor 17 under predetermined requirements.
Is corrected to calculate a corrected target air-fuel ratio coefficient KCMDM.

Next, in step S45, a limit check is performed on the corrected target air-fuel ratio coefficient KCMDM, the program is terminated, and the process returns to the main routine (FIG. 2). That is, the magnitude relationship between the KCMDM value calculated in step S44 and the predetermined upper and lower limit values KCMDMH and KCMDML is compared, and when the KCMDM value is larger than the upper limit value KCMDMH, the KCMDM value is set to the upper limit value KCMDMH, Is smaller than the lower limit value KCMDML, the KCMDM value is set to the lower limit value KCMDML.

FIG. 5 is a flow chart of the O2 processing routine executed in step S44 (FIG. 4), and this program is executed in synchronization with the generation of the TDC signal pulse.

First, in step S51, it is determined whether or not the flag FO2 is "1", and it is determined whether or not the O2 sensor 18 is activated. Whether or not the O2 sensor 18 has been activated is determined based on the elapsed time after the ignition switch is turned on, and specifically, it is determined by executing an O2 sensor activation determination routine (not shown). ..

Then, the answer to step S51 is negative (N
O), that is, when it is determined that the O2 sensor 18 has not been activated yet, the process proceeds to step S52, the timer tmRX is set to a predetermined value T2 (for example, 0.25 sec), and then the flag FVREF is set to "0". It is determined whether or not
Initial value VRRE of the target correction value VREF of the O2 sensor 18
It is determined whether or not F (hereinafter, referred to as "initial correction value") has already been set (step S53).

Then, in the first loop, step S5
Since the answer to No. 3 is affirmative (YES), the process proceeds to step S54 and the VRR stored in the storage means 5c (ROM).
The EF table is searched to calculate the initial correction value VRREF.

Specifically, as shown in FIG. 6, the VRREF table has table values VRREF0 to VREF corresponding to atmospheric pressures PA0 to PA1 detected by the PA sensor 26.
RREF2 is given in steps, and the corrected initial value VRREF is read by searching the VRREF table or calculated by the interpolation method.
As is apparent from FIG. 6, the correction initial value VRRE
F is set to a larger value as the atmospheric pressure PA is larger.

Next, in step S55, the integral term (I term) VREFI (n- of the target correction value in the previous loop is obtained.
1) is set to the correction initial value VRREF, the program is terminated, and the process returns to the main routine (FIG. 2). That is, the target correction value VREFI (n-1) of the I term is initialized and the process returns to the main routine (FIG. 2). still,
When step S53 is executed after the next loop,
As described above, since the correction initial value of the target correction value has already been set, the answer is negative (NO), and step S5
This program ends without executing 4,55.

The answer to step S51 is affirmative (Y
ES), it is determined that the O2 sensor 18 has been activated, the process proceeds to step S56, and the timer tmRX is set.
Is determined to be "0". Then, when the answer is negative (NO), the process proceeds to step S53, while when the answer at step S56 is affirmative (YES), the O2 sensor 1
It is determined that the activation of No. 8 is completed and the process proceeds to step S57, and whether the target air-fuel ratio coefficient KCMD set in step S42 or S43 (FIG. 4) is larger than the predetermined lower limit value KCMDZL (for example, 0.98). To determine. And
When the answer is negative (NO), it means that the air-fuel ratio of the air-fuel mixture is set to the lean burn state, and while this program is ended, when the answer is affirmative (YES), the routine proceeds to step S58, It is determined whether the target air-fuel ratio coefficient KCMD is smaller than a predetermined upper limit value KCMDZH (for example, 1.13). When the answer is negative (NO), it means that the air-fuel ratio of the air-fuel mixture is set to fuel rich, and while this program is ended, the answer is affirmative (Y
In the case of (ES), the air-fuel ratio of the air-fuel mixture is in a slightly rich state where NOx can be sufficiently removed (for example, A / F = 1.
4.62) should be set, and the process proceeds to step S59, where it is determined whether or not the engine is in fuel cut. When the answer is affirmative (YES), the program is terminated and the main routine (FIG. 2) is returned, while
When the answer is negative (NO), it is determined whether or not the fuel cut state was present in the previous loop (step S50). When the answer is affirmative (YES), the counter NAFC is set to a predetermined value N1 (for example, 4) (step S51), and then the counter value N1 of the counter NAFC is decremented by "1" (step S51). S52) This program ends.

On the other hand, 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 C is "0". Then, when the answer is negative (NO), the count value of the counter NAFC is decremented by "1" (step S62), and the program ends, while when the answer is affirmative (YES), the fuel cut is performed. After exiting the state, it is determined that stable fuel supply is being performed, and the routine proceeds to step S64, where O2
After performing the feedback process (step S64),
This program ends and the process returns to the main routine (Fig. 2).

FIG. 7 is a flow chart of the O2 feedback processing routine executed in step S64 (FIG. 5), and this program is executed in synchronization with the generation of the TDC signal pulse.

That is, the first and third catalytic devices 15,
In order to effectively remove NOx at 19, the O2 feedback process is performed so that the air-fuel ratio of the air-fuel mixture becomes a predetermined value slightly on the rich side of the stoichiometric air-fuel ratio (for example, corrected target air-fuel ratio coefficient KCMD = 1.005). Executed.

First, in step S81, the STUR map is searched, and the average value of the engine operating range STUR and the air-fuel ratio correction value ΔKCMD (hereinafter, this average value is referred to as a "learning value").
Read) ΔKCMDREF.

The STUR map shows the absolute pressure PBA in the intake pipe.
And a plurality of operating regions STU with respect to the engine speed NE
R and learning value ΔKC suitable for these operating regions STUR
MDREF is given, and the operating range STUR and its learning value ΔKC are searched by searching the STUR map.
MDREF is read. The learning value ΔKCMDRE
As will be described later, F is calculated for each operating region based on a learning value calculation formula and stored in the storage means 5c.

Next, at step S82, the operating range STUR (n) for the current loop and the operating range ST for the previous loop are set.
It is determined whether or not UR (n-1) is equal.

When the answer is negative (NO), that is, when the operation range STUR is different between the previous loop and the current loop, the air-fuel ratio correction value ΔKCMD is set to the learning value ΔKCMDREF of the operation range STUR (n) during the current loop. And set to step S95.

On the other hand, the answer to step S82 is affirmative (YE
If S), the process proceeds to step S84 and the thinning-out variable NI
It is determined whether VR is “0”. This decimation variable NIV
R is the same as the NITDC (see FIG. 3) described above, TDC
This is a variable that is subtracted each time the signal pulse is generated by the thinned-out TDC number NI set according to the engine operating state, and is initially "0", so the answer to step S84 is affirmative (YES), and step S85 Proceed to.

When the answer to step S84 is negative (NO) in the subsequent loop, the process proceeds to step S86, and the value obtained by subtracting the thinning-out TDC number NI (for example, 1) from the thinning-out variable NIVR is added to the new thinning-out variable. After setting to NIVR, the process proceeds to step S96 described later.

Therefore, in the step S85, O2
It is determined whether or not the output voltage VO2 of the sensor 18 is smaller than a predetermined lower limit value VL (for example, 0.3V). And
When the answer is affirmative (YES), the air-fuel ratio in which the air-fuel ratio of the air-fuel mixture is in a slightly rich state (target air-fuel ratio coefficient KCMD =
1.005), the process proceeds to step S68 when it is determined that the output voltage V has shifted to the lean side, and if the answer is negative (NO), the process proceeds to step S87 and the output voltage V of the O2 sensor 18 is increased.
It is determined whether O2 is larger than a predetermined upper limit value (for example, 0.8). When the answer is affirmative (YES), it is determined that the air-fuel ratio of the air-fuel mixture is deviating from the stoichiometric air-fuel ratio to the rich side, and the routine proceeds to step S88.

Next, in step S88, the KVP map, KVI map, KVD map, and NIVR map are searched to change the O2 feedback control, that is, the proportional term (P term) coefficient KVP and the integral term (I term) coefficient KVI. ,
Differential term (D term) coefficient KVD and the decimation number NIVR
Is calculated. KVP map, KVI map, KVD
Maps and NIVR maps are the KP map, K
Similar to the I map and the like (see FIG. 3), a predetermined map value is given for each of a plurality of engine operating regions determined by the engine speed NE and the intake pipe absolute pressure PBA, and the engine operation is performed by searching these maps. A map value corresponding to the state is read out or calculated by an interpolation method.

Next, in step S89, the thinning-out variable NIV is set.
After setting R to the NIVR value calculated in step S88, the process proceeds to step S90, and the step S54 is performed.
Deviation Δ between the corrected initial value VRREF calculated in (FIG. 5) and the output voltage VO2 of the O2 sensor 18 in the current loop.
Calculate V (n).

Next, in step S91, equations (5)-
Based on (7), the target correction values VREFP (n), VREFI (n), V of each correction term, that is, P term, I term, D term,
After calculating REFD (n), these correction terms are added to calculate the target correction value VREF (n) in the O2 feedback based on Expression (8).

VREFP (n) = ΔV (n) × KVP (5) VREFI (n) = VREF + ΔV (n) × KV1 (6) VREFD (n) = (ΔV (n) -ΔV (n-1) ) × KVD (7) VREF (n) = VREFP (n) + VREFI (n) + VREFD (n) (8) Next, in step S92, a limit check of VREF (n) is performed.

That is, VREF (n) and VREFI
(N) value is compared with the predetermined upper and lower limit values VREFH, VREFI, and as a result, the VREF (n) value and VRE are compared.
When the FI (n) value exceeds the upper limit value VREFH, the predetermined upper limit value VREFH is set, and the predetermined lower limit value VREFI is set.
When it is smaller, it is set to the predetermined lower limit value VREFI.

In step S93, the air-fuel ratio correction value ΔKC
MD is calculated. The air-fuel ratio correction value ΔKCMD is specifically calculated by searching the ΔKCMD table as shown in FIG. That is, the ΔKCMD table is a table value Δ for the target correction values VREF0 to VREF5.
KCMD0 to ΔKCMD3 are given, and the air-fuel ratio correction value ΔKCMD is read by searching the ΔKCMD table or calculated by the interpolation method. As is clear from FIG. 8, the ΔKCMD value is generally set to a larger value as VREF (n) has a larger value. Further, since the VREF value is subjected to the limit check in step S92, the ΔKCMD value is also set to a value within a predetermined upper and lower limit value.

Next, in step S94, the learning value ΔKCMDREF is calculated based on the learning value calculation formula shown in the mathematical expression (9).

[0099]

[Equation 1] Here, CREF is 1 to 6 depending on the operating state of the engine.
Variable that is set to an appropriate value within the range of 5536, ΔKCM
DREF (n-1) is the previously calculated value of the learning value KREF, and the air-fuel ratio correction value ΔKCMD is the previously learned value ΔKCMDR.
By performing learning calculation with EF (n-1), the learning value ΔKCMDREF of each operating region STU is updated,
Always appropriate target air-fuel ratio without deterioration over time, that is, NOx
The air-fuel ratio feedback control can be performed with an air-fuel ratio that can be sufficiently removed.

Next, at step S95, the learned target value ΔKCMDREF is added to the target air-fuel ratio coefficient KCMD calculated at step S42 (FIG. 4) to calculate a corrected target air-fuel ratio coefficient KCMDM (for example, 1.005). This program ends.

The answer to step S87 is negative (N
O), that is, the output voltage VO2 of the O2 sensor 18 is V
When L ≦ VO2 ≦ VH, the O2 feedback control is prohibited, and in steps S96 to S98, the ΔV value (deviation between the correction initial value VRREF and the output value of the O2 sensor 17),
The present value is set equal to the previous value for each of the target correction value VREF and the air-fuel ratio correction value ΔKCMD, and the program ends. As a result, when the air-fuel ratio of the air-fuel mixture can be maintained at an air-fuel ratio that can sufficiently remove NOx, unnecessary O2 feedback control is prohibited, and the control system can be kept in good condition. That is, the air-fuel ratio of the air-fuel mixture can be maintained stable.

Next, the control procedure of the secondary air supply system 22 will be described in detail.

FIG. 9 is a flow chart showing the procedure for controlling the secondary air.

First, in step S101, output values from various sensors such as the PBA sensor 8, the TW sensor 10, the NE sensor 11, the TCT sensor 16 and the TCO sensor 23 are read. Next, switching valve control, which will be described later, is executed to determine whether the position setting of the switching valve 31 and the lift permission of the valve body 34 of the control valve 30 are permitted (step S102). Then, it is determined whether the flag FSA is "0" by executing the switching valve control, and it is determined whether the lift of the valve element 34 of the control valve 30 is permitted (step S103). When the answer is affirmative (YES), the reference lift command value LCMDA of the L sensor 38 is set to "0" (step S10).
4) While ending this program, when the answer to step S103 is negative (NO), the reference lift command value LCMDA is calculated according to the operating state of the engine and the like (step S105), and the output value of the LAFR sensor 50 is calculated. Based on the feedback control of the secondary air (step S10)
6). Then, after that, the reference lift command value LCMD
A correction value KDAF of A is calculated (step S107), and then a final lift command value LCMDAF is calculated based on the LCMDA value and the KDAF value (step S10).
8) End this program.

Each of the above processing steps will be described in detail below.

Switching Valve Control (Step S102) FIG. 10 is a flowchart of the switching valve control routine executed in step S102 (FIG. 9), and this program is executed in synchronization with the generation of the TDC signal pulse.

First, in step S111, it is determined whether or not the engine is in the starting mode, and the answer is affirmative (YE
S), the timer tmSA is set to a predetermined value T3 (for example, 6).
0) (step S112), then flag FS
A is set to "0" (step S113), and the control valve 30
This program is terminated by prohibiting the lift permission of the valve body 34 of FIG.

On the other hand, the answer to step S111 is negative (N
When it is O), the engine is in the basic mode, and it is determined whether or not the chamber pressure PC is larger than a predetermined lower limit value PCL (step S114). Then, when the answer is negative (NO), the process proceeds to step S113, while when the answer is affirmative (YES), it is determined whether or not the engine cooling water temperature TW is larger than a predetermined value TWL (for example, 50) ( Step S115).

When the answer is affirmative (YES), it means that the engine has been warmed up, and the timer tmSA is set to "0" (step S116), and the process proceeds to step S118.

On the other hand, the answer to step S115 is negative (N
O), the catalyst bed temperature TCT of the first catalyst device 15
(Detected by the TCT sensor 16) is a predetermined value TCL
It is determined whether it is larger than 1 (for example, 20) (step S117). If the answer is negative (NO), the process proceeds to step S113, while the answer is affirmative (YE).
If S), the process proceeds to step S118.

Next, in step S118, the timer tm
It is determined whether SA is "0". When the answer is affirmative (YES), the catalyst bed temperature T of the second catalyst device 20 is
It is determined whether or not CO (detected by the TCO sensor 23) is larger than a predetermined value TCL2 (for example, 250 ° C.) (step S119). And the answer is negative (N
When the answer is affirmative (YES), it is determined whether or not the flag FSA is "0" (step S120). When the answer is affirmative (YES), the flag FFDS is set to "0" so that the port position of the switching valve 31 makes the control valve 30 and the first secondary air introduction hole 21 communicate with each other. After issuing the command (step S121), the flag FSA is set to "1" to permit the lift of the valve element 34 (step S12).
2) End this program.

On the other hand, the answer to step S120 is negative (N
O), that is, when the flag FSA is set to "1" and the lift of the valve element 34 is permitted, step S12.
In step 3, it is determined whether the flag FFDS is "0".
When the answer is affirmative (YES), the control valve 30
When the answer is negative (NO), the flag FFDS is set to "0" and the control valve 30 is activated. The switching valve 31 is switched so that the first secondary air introduction hole 21 and the first secondary air introduction hole 21 are in communication with each other (step S12).
4) The program ends.

When the answer to step S118 is negative (NO), it is determined whether the flag FSA is "0" (step S125). And the answer is negative (N
In the case of O), the flag FSA is "1", and this program is terminated as it is, while step S125.
If the answer is YES (YES), it is determined that the engine has not warmed up and the engine temperature is low, and the flag FFDS is set to "1".
After the control valve 30 and the second secondary air introduction hole 42 are set to be in communication with each other (step S126), the flag FS is set.
A is set to "1" to permit the lift of the valve element 34 (step S127), the secondary air supply to the first catalyst device 15 is commanded, and this program is ended.

Calculation of Basic Lift Command Value LCMDA (Step S105) FIG. 11 shows the LC executed in step S105 (FIG. 9).
It is a flowchart of the MDA calculation routine, and this program is executed in synchronization with the generation of the TDC signal pulse.

First, in step S130, the detected air-fuel ratio coefficient KACTR of the LAFR sensor 50 is set to the lower limit predetermined value KACT required to supply the secondary air amount to the second catalyst device 20.
It is determined whether or not it is larger than L. When the answer is negative (NO), the basic lift command value LCMDA is set to "0".
To prohibit the lift of the valve element 34 of the control valve 30 (step S131), and the first secondary air introduction hole 2 for the secondary air is set.
The supply to 1 is prohibited and this program ends.

On the other hand, the answer in step S130 is affirmative (YE
If S), the process proceeds to step S132, and the flag FVTE
It is determined whether C is "0" to determine whether the valve timing of the intake valve or the exhaust valve is set to the low speed V / T. When the answer is affirmative (YES), the valve timing is set to the low speed V / T, and the process proceeds to step S133, where the low speed V / T LMAPA is used.
After searching the map to calculate the basic lift command value LMAPA suitable for the low speed V / T, the process proceeds to step S135.
On the other hand, when the answer to step S132 is negative (NO), the valve timing is set to the high speed V / T, and the flow advances to step S134 to proceed to the high speed V / T LMA.
After searching the PA map to calculate the basic lift command value LMAPA suitable for the high speed V / T, the process proceeds to step S135.

Specifically, as shown in FIG. 12, the LMAPA map is a matrix of map values LMAPA0000 to LMAPA16 for intake pipe absolute pressures PBA00 to PBA16 and engine speeds NE00 to NE19.
19 is given, and the basic lift command value LMAPA is read by searching this LMAPA map or calculated by the interpolation method. That is, high speed V
LMAPA map suitable for / T and L suitable for low speed V / T
The MAPA map and the storage means 5C (ROM) are stored in advance, and the basic lift command value LMAPA is calculated by searching either the high speed V / T LMAPA map or the low speed V / T MAPA map. ..

Next, in step S135, the KPAEX table is searched to calculate the atmospheric pressure correction value KPAEX.

The KPAEX table is specifically shown in FIG.
As shown in the table, table values KPAEX0 to KPAEX2 are given to the atmospheric pressures PA0 to PA4, and the atmospheric pressure correction value KPAEX is read by searching the KPAEX table or calculated by an interpolation method. .. The KPAEX value is set so that the table value KPAEX2 is "1.0", and therefore K
The PAEX value is set to a value smaller than "1.0".

Next, in step S114, equation (10)
The basic lift command value LCMDA of the valve element 34 is calculated based on the above, and this program ends.

LCMDA = LMAPA × KPAEX (10) Secondary air feedback control (step S106) In the secondary air feedback control, a secondary air flow rate based on the output value of the LAFR sensor 50 is obtained by the same method as that shown in FIG. Take control. That is, a feedback control routine similar to that of FIG. 3 is executed, and when the predetermined feedback condition is not satisfied, the flag FLAFFB is set to "0" to inhibit the feedback control, while when the predetermined feedback condition is satisfied, The flag FLAFFB is set to "1" and the air-fuel ratio correction coefficient KL is set.
Calculate AFR and command execution of feedback control.

The air-fuel ratio correction coefficient KLAFR is calculated by a method substantially similar to the above-mentioned KLAFF calculation routine (FIG. 3). That is, step S21 (FIG. 3)
Since the method of calculating the KLAFF value is the same as the calculation method of the KLAFF value except that the ΔKAF value is calculated based on the mathematical expression (11), the detailed description thereof will be omitted.

ΔKAF = KACT (n−1) −KCMD (n−1) (11) Calculation of Lift Correction Value KDAF (Step S107) FIG. 14 shows the KD executed in step S107 (FIG. 9).
It is a flowchart of the AF calculation routine, and this program is executed in synchronization with the generation of the TDC signal pulse.

First, in step S141, the mathematical expression (1
Based on 2), the target air-fuel ratio coefficient KT of the LAF sensor 17 in the start-up mode in which the air-fuel ratio at start-up is in a rich state
A deviation ΔKCMDRA between the WLAF and the target air-fuel ratio coefficient KCMDR in the second catalyst device 20 is calculated.

ΔKCMDRA = KTWLAF−KCMDR (12) Here, the KCMDR value is set to a predetermined value such that the air-fuel ratio of the air-fuel mixture in the second catalyst device 20 is in a lean state, for example, 1.02. ..

Next, in step S142, the KDAFC table is searched to calculate the correction value KDAFC of the secondary air amount with respect to the stoichiometric air-fuel ratio. Specifically, the KDAFC table, as shown in FIG. 15, includes deviations ΔKCMDRA0 to ΔΔ.
Table values KDAFC0 to KD for KCMDRA4
AFC2 is given, and the KDAFC value is the KDAFC value.
It is read by searching the FC table or detected by the interpolation method.

Next, at step S143, it is judged if the deviation ΔKAF between the corrected target air-fuel ratio coefficient KCMDM of the LAFF sensor 17 and the detected air-fuel ratio coefficient KACT is smaller than a predetermined value ΔKAFL (for example, -0.025). .. When the answer is negative (NO), the detected air-fuel ratio coefficient KACT is relatively close to the corrected target air-fuel ratio coefficient KCMDM, and it is determined that the actual air-fuel ratio of the air-fuel mixture is not so rich, and the correction basic value KDAFA is set. The value is set to "1.0" (step S144), and the process proceeds to step S146.

On the other hand, the answer in step S143 is affirmative (YE
In the case of S), the correction basic value KDAFA is calculated according to the enriched state of the air-fuel ratio as shown in the equation (13), and the routine proceeds to step S146.

[0129]

[Equation 2] In this case, | ΔKAF |> | ΔKCMDRA |, so KDAFA> 1.0, and the secondary air amount QCMD
Will be corrected in the increasing direction.

Then, in step S146, the mathematical expression (1
As shown in 4), the KDAFC value and the KDAFA
This value is multiplied to calculate the lift correction value KDAF, and this program ends.

KDAF = KDAFC × KDAFA (14) Calculation of final target lift value LCMDAF (step S1
08) FIG. 16 shows the LC executed in step S108 (FIG. 9).
It is a flowchart of the MDAF calculation routine, and this program is executed in synchronization with the generation of the TDC signal pulse.

First, in step S151, QCMDA
The table is searched to calculate the basic secondary air amount QCMDA. In the QCMDA table, specifically, as shown in FIG. 17, table values QCMDA0 to QCMDA6 are given to the basic lift command values LCMDA0 to LCMDA6, and the basic secondary air amount QCMDA is the QCMDA table.
It is read by searching the table A or calculated by the interpolation method.

Next, the secondary air amount Q is calculated based on the equation (15).
CMD is calculated (step S152).

QCMD = QCMDA × KDAF × KLAFR (15) Next, in step S153, the LCMDAF table is searched to calculate the final target lift value LCMDAF.

The LCMDAF table is specifically shown in FIG.
As shown in FIG. 8, table values LCMDAF0 to LCMDAF4 are given to the secondary air amounts QCMD0 to QCMD6, and the final target lift value LCMDAF is read by searching this LCMDAF table, or by an interpolation method. It is calculated.

Next, in step S154, it is determined whether or not the final target lift value LCMDAF is larger than the predetermined upper limit value LHL. When the answer is affirmative (YES), the predetermined upper limit value LHL is set to the final target lift value LCMD.
AF is set (step S155), and this program ends. On the other hand, when the answer to step S154 is negative (NO), the final target lift value LCMDAF is the predetermined lower limit value L.
It is determined whether it is smaller than LL (step S15).
6). When the answer is negative (NO), the LCMDAF value calculated in step S153 is directly used as the final target lift value, and when the answer is affirmative (YES), the predetermined lower limit value LLL is set as the final target lift value. Value LCMD
AF is set (step S157), and this program ends.

FIG. 19 is a characteristic diagram showing the purification characteristics of HC, CO and NOx, where the horizontal axis is the (corrected) target air-fuel ratio coefficient KCMD (M) and the vertical axis is the purification rate (η). Also, in the figure, W indicates a predetermined region near the stoichiometric air-fuel ratio called a window.

In the exhaust gas purifying method according to the first conventional example described in the section [Prior Art], the target air-fuel ratio (corrected) target air-fuel ratio coefficient KCMD (M) is "1.
According to the present invention, the exhaust gas is purified at the point "0", while the NOx purification rate (η) is maximized in the vicinity of the point A for the first and second catalyst devices 15 and 19. Since the corrected target air-fuel ratio KCMDM is set to (for example, KCMD
M = 1.005) NOx improves and NO per unit volume
The purification rate for x can be improved. That is,
As in the first conventional example, all of the first to third catalytic devices are used as a three-way catalyst without introducing secondary air, and the air-fuel ratio of the air-fuel mixture is accurately set to the theoretical air-fuel ratio. NO
Compared with the case where x is removed, the present invention sets the air-fuel ratio of the air-fuel mixture in the first and second catalyst devices 15 and 19 to be slightly rich, so that the NOx purification rate per unit volume can be improved. Therefore, even if the volume ratio of the three-way catalyst becomes about (2/3) as compared with the first conventional example, it can be expected that the same purification efficiency can be obtained.

On the other hand, by making the first and second catalytic devices 15 and 19 slightly rich as described above, HC and CO
Since the purification rate of B is inferior to that of the first conventional example because the purification rate is point B'and point C ', respectively, secondary air is supplied to the second catalyst device 20 and mixed. For example, the target air-fuel ratio coefficient KCM is set so that the air-fuel ratio becomes lean.
By setting DR to "0.98", points B and C
As shown in, the purification rate of HC and CO is improved as compared with the first conventional example.

As described above, the catalyst device of the first conventional example is divided into about three equal parts, and the first and second catalyst devices 15 and 19 are used as the three-way catalyst, while the second catalyst device 20 is oxidized. By using it as a catalyst, the purification rate of exhaust gas can be improved. The volume ratio of the three-way catalyst (first and second catalyst devices 15, 19) to the oxidation catalyst (second catalyst device 20) is preferably 1.8: 1 to 2.2: 1. This is supported by the applicant's experiments.

FIG. 20 shows the corrected target air-fuel ratio coefficient K of the present invention.
It is the figure which showed the distribution of CMDM in comparison with the 2nd prior art example which uses an O2 sensor (refer to the paragraph of [prior art]).

In the figure, the horizontal axis represents (corrected) target air-fuel ratio coefficient KC.
MD (M), the vertical axis represents frequency.

In the O2 sensor, the target air-fuel ratio coefficient KCMD
Is merely outputting the inversion signal at the position of "1.0", and the actual fluctuation range of the air-fuel ratio is large, whereas in the present invention, the target air-fuel ratio KCMD of the LAFF sensor 17 is made slightly rich. The distribution of the corrected target air-fuel ratio coefficient KCMDM is also within the window W on the rich side. That is, by making the target air-fuel ratio of the LAFF sensor 17 slightly rich as in the present invention, the conventional O2 sensor is NO.
It is possible to improve the NOx purification rate without generating the shaded area where the removal of x is not sufficiently performed.

Furthermore, in the above embodiment, LAFR
Since the secondary air amount is feedback-controlled based on the output value of the sensor 50, the secondary air amount can be optimally controlled, and deterioration of the second catalyst device 20 due to the oxidizing atmosphere can be suppressed to the minimum. .

It is needless to say that the present invention is not limited to the above-mentioned embodiments and can be modified within the scope not departing from the gist. For example, as shown in FIG. 21, the first catalyst device 44 whose volume ratio is 1.8 to 2.2 times that of the second catalyst device 20 is installed in the first exhaust gas concentration sensor 17 and the second exhaust gas concentration sensor. 18 and the first catalyst device 44
You may comprise so that it may be possible to remove NOx sufficiently.

[0146]

As described above in detail, the exhaust gas purifying apparatus for an internal combustion engine according to the present invention includes a first catalytic device arranged in the exhaust passage of the internal combustion engine for purifying harmful components in the exhaust gas, and A first catalyst device having an output characteristic that is substantially proportional to the concentration of exhaust gas disposed in the exhaust passage upstream of the first catalyst device;
First exhaust gas concentration sensor, operating state detecting means for detecting an operating state of the engine including at least the engine speed and the load state of the engine, and a first air-fuel ratio calculation based on the detection result of the operating state detecting means. Target air-fuel ratio calculation means, a second exhaust gas concentration sensor disposed in the exhaust passage downstream of the catalyst device and having an output signal inverted near the target air-fuel ratio, and the second exhaust gas concentration sensor The enrichment means for setting the target air-fuel ratio to a slightly richer state than the stoichiometric air-fuel ratio based on the output value of, and the air-fuel ratio of the air-fuel mixture detected by the first exhaust gas concentration sensor are set by the enrichment means. And a control means for performing feedback control to the target air-fuel ratio, and further, a second catalyst device and a third catalyst device are sequentially connected in series in the exhaust passage downstream of the second exhaust gas concentration sensor. And a third exhaust gas concentration sensor is provided downstream of the third catalyst device and provided in an exhaust passage between the second catalyst device and the third catalyst device. The secondary air supply means and the air flow rate control means for feedback-controlling the secondary air flow rate from the secondary air supply means based on the output value of the third exhaust gas concentration sensor. The air-fuel ratio of the air-fuel mixture in the second catalyst device and the third catalyst device is set to a slightly rich state so that NOx can be effectively removed. Further, the second air supply means makes the second catalyst device lean In this state, HC and CO can be effectively removed, the purification rate can be improved, the deterioration of the catalyst device can be suppressed, and the durability can be improved. Further, since the secondary air flow rate is feedback-controlled by the air flow rate control means based on the output value of the third exhaust gas concentration sensor, it is possible to minimize deterioration of the catalyst device due to the oxidizing atmosphere. Therefore, the deterioration of durability can be avoided as much as possible.

Further, since the third catalyst device is provided in the exhaust passage between the second air-fuel ratio sensor and the secondary air supply means, the secondary air is removed by the pulsation of exhaust gas or the like. It is possible to avoid adversely affecting the exhaust concentration sensor of No. 2 and to reduce the volume of the first catalytic device to accelerate the activation of the first catalytic device at low temperature, and to further reduce harmful components. Effective purification is possible.

Specifically, the air flow rate control means controls the air-fuel ratio of the air-fuel mixture in the second catalyst device to a leaner state than the stoichiometric air-fuel ratio based on the output value of the third exhaust gas concentration sensor. By doing so, the above-mentioned effective removal of harmful components and prevention of deterioration of the catalyst device can be achieved.

Further, there is provided valve timing switching means capable of switching the valve timing of the intake and exhaust valves of the engine to at least a low speed valve timing suitable for a low rotation range and a high speed valve timing suitable for a high rotation range, and
The secondary air supply means calculates the basic air flow rate in accordance with the valve timing setting state of the valve timing switching means and the detection result of the operating state detection means, and the third exhaust gas concentration sensor. Correction coefficient calculation means for calculating an air-fuel ratio correction coefficient on the basis of the output value, and at least the basic air flow rate calculated by the air flow rate calculation means and the air-fuel ratio correction coefficient calculated by the correction coefficient calculation means. The desired secondary air flow rate can be easily determined by including the air flow rate determination means for determining the secondary air flow rate.

[Brief description of 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 flowchart showing a main routine of air-fuel ratio feedback control of an internal combustion engine according to the present invention.

FIG. 3 is a flowchart of a KLAFF calculation routine.

FIG. 4 is a flowchart of a KCMDM calculation routine.

FIG. 5 is a flowchart of an O2 processing routine.

FIG. 6 is a VRREF table.

FIG. 7 is a flowchart of an O2 feedback processing routine.

FIG. 8 is a ΔKCMD table diagram.

FIG. 9 is a flowchart showing a control procedure of secondary air.

FIG. 10 is a flowchart of a switching valve control routine.

FIG. 11 is a flowchart of an LCMDAR calculation routine.

FIG. 12 is an LMPA map diagram.

FIG. 13 is a KDAEX table diagram.

FIG. 14 is a flowchart of a KDAF calculation routine.

FIG. 15 is a KDAFC table diagram.

FIG. 16 is a flowchart of an LCMDAF calculation routine.

FIG. 17 is a QCMDA table diagram.

FIG. 18 is an LCMDAF table diagram.

FIG. 19 is an air-fuel ratio distribution chart showing the air-fuel ratio characteristics during feedback control in comparison with a conventional example.

FIG. 20 is a purification characteristic diagram showing a purification rate of exhaust gas.

FIG. 21 is a configuration diagram of main parts of another embodiment of the present invention.

[Explanation of symbols]

1 engine 5 ECU (target air-fuel ratio calculation means, enrichment means, control means, secondary air supply means, valve timing switching means,
Air flow rate calculation means, correction coefficient calculation means, air flow rate determination means) 8 PBA sensor (operating state detection means) 11 NE sensor (operating state detection means) 14 Exhaust pipe (exhaust passage) 15 First catalyst device 17 LAFF sensor ( First exhaust gas concentration sensor) 18 O2 sensor (second exhaust gas concentration sensor) 19 Second catalyst device 20 Third catalyst device 22 Secondary air supply system (secondary air supply means) 50 LAFR sensor (third exhaust gas sensor) Exhaust concentration sensor)

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Office reference number FI technical display location F02D 41/14 310 P 7813-3G

Claims (4)

[Claims] 1. A first catalyst device arranged in an exhaust passage of an internal combustion engine to purify harmful components in exhaust gas, and the first catalyst device.
A first exhaust gas concentration sensor having an output characteristic substantially proportional to the exhaust gas concentration arranged in the exhaust passage on the upstream side of the catalyst device, and an engine operating condition including at least the engine speed and the engine load condition. The operating state detecting means for detecting, the first target air-fuel ratio calculating means for calculating the target air-fuel ratio based on the detection result of the operating state detecting means, and the exhaust passage on the downstream side of the catalyst device are provided. A second exhaust gas concentration sensor in which an output signal is inverted in the vicinity of the target air-fuel ratio, and a rich that sets the target air-fuel ratio to a slightly rich state from the theoretical air-fuel ratio based on the output value of the second exhaust gas concentration sensor. Enriching means and control means for feedback controlling the air-fuel ratio of the air-fuel mixture detected by the first exhaust gas concentration sensor to the target air-fuel ratio set by the enriching means, and A second catalyst device is disposed in the exhaust passage downstream of the second exhaust gas concentration sensor, and a third exhaust gas concentration sensor is disposed downstream of the second catalyst device. Secondary air supply means provided in the exhaust passage on the upstream side of the catalyst device and on the downstream side of the second exhaust gas concentration sensor, and the secondary air based on the output value of the third exhaust gas concentration sensor. An exhaust gas purifying apparatus for an internal combustion engine, comprising: an air flow rate control unit that feedback-controls a secondary air flow rate from a supply unit. 2. The internal combustion engine according to claim 1, wherein a third catalyst device is provided in an exhaust passage between the second air-fuel ratio sensor and the secondary air supply means. Exhaust gas purification device. 3. The air flow control means controls the air-fuel ratio of the air-fuel mixture in the second catalyst device to a leaner state than the stoichiometric air-fuel ratio, based on the output value of the third exhaust gas concentration sensor. An exhaust gas purifying apparatus for an internal combustion engine according to claim 1 or 2, characterized in that 4. A valve timing switching means capable of switching a valve timing of an intake and exhaust valve of an engine to at least a low speed valve timing suitable for a low rotation region and a high speed valve timing suitable for a high rotation region, The next air supply means calculates the basic air flow rate according to the setting state of the valve timing by the valve timing switching means and the detection result of the operating state detection means, and the output of the third exhaust gas concentration sensor. Correction coefficient calculation means for calculating an air-fuel ratio correction coefficient based on the value, and secondary air based on at least the basic air flow rate calculated by the air flow rate calculation means and the air-fuel ratio correction coefficient calculated by the correction coefficient calculation means 4. An air flow rate determining means for determining a flow rate, comprising any one of claims 1 to 3. Exhaust gas purification system of an internal combustion engine according to.