JPH08128347A - Air-fuel ratio control device - Google Patents

Abstract

PURPOSE: To maintain excellent exhaust gas purifying performance by using catalytic converter rhodium to reduction in a time constant of an oxygen concentration sensor without requiring complicated control in an air-fuel ratio control device by considering the degradation of the oxygen concentration sensor. CONSTITUTION: An air-fuel ratio control device is provided with an air-fuel ratio control means 20 to control the air-fuel ratio on the basis of output of an oxygen concentration sensor 8 arranged upstream of catalytic converter rhodium so that the target air-fuel ratio is realized, a time constant detecting means to detect a present time constant of the oxygen concentration sensor 8 and a target air-fuel ratio changing means to largely change the target air-fuel ratio to the rich side from the stoicliometric air-fuel ratio as the present time constant detected by the time constant detecting means becomes large.

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 in consideration of deterioration of an oxygen concentration sensor for detecting oxygen concentration in exhaust gas in an internal combustion engine.

[0002]

2. Description of the Related Art A number of air-fuel ratio controls have been proposed which adjust the fuel injection amount with respect to the intake air amount based on the output of an oxygen concentration sensor. In general, an oxygen concentration sensor cannot change its output instantaneously with respect to a change in oxygen concentration in exhaust gas due to its characteristics, and has a certain response delay. If such a response delay increases with deterioration, the output of the oxygen concentration sensor currently used for fuel injection amount correction is based on the air-fuel ratio of the air-fuel mixture long before, which is good for combustion or exhaust gas purification. Air-fuel ratio control may not be realized.

JP-A-57-124248 discloses an oxygen concentration sensor that intentionally increases or decreases the fuel injection amount in all cylinders simultaneously to change the air-fuel ratio on a cycle-by-cycle basis based on a predetermined fluctuation waveform, and changes accordingly. It is described that the current response delay (hereinafter, a time constant) of the oxygen concentration sensor is grasped by comparing the output fluctuation waveform of (1) with a predetermined fluctuation waveform of the air-fuel ratio.

[0004]

The above-mentioned prior art merely detects the time constant of the oxygen concentration sensor, and when the time constant exceeds a predetermined value, the oxygen concentration sensor should be replaced. Although it is possible to warn the driver, this predetermined value is generally set relatively loosely because the oxygen concentration sensor cannot be replaced frequently, and various set values in the air-fuel ratio control are set to the oxygen concentration sensor. Since it is determined based on the initial time constant of, the deterioration of the air-fuel ratio control to some extent is sacrificed while the initial time constant deteriorates to a predetermined value.

It is also possible to correct the output of the oxygen concentration sensor to the output at the time of the initial time constant based on the detected time constant, but this is to predict the future,
It is not accurate. On the other hand, various set values in the air-fuel ratio control can be reset according to the current time constant of the oxygen concentration sensor. However, it is not very realistic because the control becomes very complicated.

Therefore, the first object of the present invention is to maintain good exhaust gas purification performance by using a three-way catalyst against deterioration of the time constant of the oxygen concentration sensor without requiring complicated control. To provide a simple air-fuel ratio control device.

A second object of the present invention is to provide an air-fuel ratio control device which can realize good air-fuel ratio control against deterioration of the time constant of the oxygen concentration sensor without requiring complicated control. It is to be.

[0008]

According to a first aspect of the present invention, there is provided an air-fuel ratio control device for adjusting an air-fuel ratio based on an output of an oxygen concentration sensor arranged upstream of a three-way catalyst so that a target air-fuel ratio is realized. Air-fuel ratio control means for controlling, time constant detection means for detecting the current time constant of the oxygen concentration sensor, the larger the current time constant detected by the time constant detection means, the more the target air-fuel ratio And a target air-fuel ratio changing means for largely changing the stoichiometric air-fuel ratio to the rich side.

According to a second aspect of the present invention, there is provided an air-fuel ratio control device for detecting the current time constant of an oxygen concentration sensor, and various set values in the air-fuel ratio control for the oxygen concentration sensor. A set value determining means for determining so that a good air-fuel ratio control is realized based on a predetermined time constant larger than the initial time constant, and the current time constant detected by the time constant detecting means for the output of the oxygen concentration sensor. Based on
Correction means for correcting the output when the oxygen concentration sensor reaches the predetermined time constant, and performing air-fuel ratio control using the output of the oxygen concentration sensor corrected by the correction means. Is characterized by.

[0010]

In the air-fuel ratio control device according to the first aspect, the air-fuel ratio is controlled based on the output of the oxygen concentration sensor so that the air-fuel ratio control means realizes the target air-fuel ratio, and the correction means detects the time constant. As the current time constant detected by the means is larger, the target air-fuel ratio is changed from the stoichiometric air-fuel ratio to the rich side to a large extent, so that the larger the time constant of the oxygen concentration sensor is, the larger the air-fuel ratio fluctuation range is. Since the center of fluctuation is on the rich side, the characteristics of the three-way catalyst can maintain the purification rate of NOx and HC higher than when the fluctuation range of the air-fuel ratio becomes large when the center of fluctuation is the stoichiometric air-fuel ratio. it can.

Further, in the air-fuel ratio control apparatus according to the second aspect of the present invention, the set value determining means allows the various set values of the air-fuel ratio control to be good based on predetermined time constants larger than the initial time constant of the oxygen concentration sensor. It is determined that the control is realized, and the correction unit outputs the output of the oxygen concentration sensor to the output when the oxygen concentration sensor reaches the predetermined time constant based on the current time constant detected by the time constant detection unit. For the purpose of correction, even if the time constant of the oxygen concentration sensor changes, good air-fuel ratio control is always realized when the time constant is the predetermined time constant.

[0012]

1 is a schematic diagram of an internal combustion engine equipped with an air-fuel ratio control device according to the present invention. In the figure, 1
Is a four-cylinder engine, and its ignition order is the first cylinder #
The order is 1st, 3rd cylinder # 3, 4th cylinder # 4, 2nd cylinder # 2. 2 is a surge tank, and 4 intake pipes 3
Is connected to each cylinder of the engine 1. Also,
An upstream intake passage 5 in which a throttle valve 4 is arranged is connected to an upstream side of intake of the surge tank 2. A fuel injection valve 6 for injecting fuel into each cylinder is provided in each intake pipe 3.
Is arranged.

An exhaust pipe 7 is connected to each cylinder of the engine 1, and an oxygen concentration in the exhaust gas is provided in a collecting portion 7a of the exhaust pipe 7 (provided that the oxygen concentration in the exhaust gas is rich when the air-fuel mixture is rich. Oxygen concentration sensor 8 for detecting the negative value) is installed. Furthermore, a three-way catalytic converter 9 is arranged on the exhaust pipe 7 downstream of the oxygen concentration sensor 8, and on the downstream side thereof, a normal concentration battery type using a zirconia solid electrolyte whose output suddenly changes near the stoichiometric air-fuel ratio is used. The rear oxygen sensor 10 is installed. Reference numeral 20 denotes a control device that is in charge of air-fuel ratio control based on the outputs of the oxygen concentration sensor 8 and the rear oxygen sensor 10, and each sensor for grasping the engine operating state, for example, an air flow meter for measuring the intake air amount. 21, a rotation sensor 22 for measuring the engine speed, a cooling water temperature sensor 23 for measuring the cooling water temperature as the engine temperature, etc. are connected.

In the air-fuel ratio control performed by the control device 20, a deposit is attached to the collision position of the fuel injected from the fuel injection valve 6, for example, the back of the intake valve, and the fuel is injected from the fuel injection valve 6. Considering that part of the fuel that has been absorbed by the deposit is not supplied to the cylinder and that part of the fuel that has been absorbed by the deposit is discharged from the deposit and supplied to the cylinder, The fuel injection amount is controlled so as to realize the fuel ratio.
The following equation is used to describe the fuel behavior. fw _{k + 1} = Pfw _k + Rfi _k fc _k = (1-P) fw _k + (1-R) fi _k where fw is the amount of fuel absorbed in the deposit, f
i is the fuel injection amount, fc is the amount of fuel supplied to the cylinder, coefficient P is the ratio of the fuel absorbed in the deposit that is not supplied to the cylinder, and coefficient R is the amount of fuel injected into the deposit The respective percentages are shown.

This air-fuel ratio control is generally known by solving the above equation using modern control theory and the like, and is supplied into the cylinder based on the intake air amount by the output of the oxygen concentration sensor 8. The fuel amount fc is calculated and the fuel injection amount fi for realizing the target air-fuel ratio is determined. The rear oxygen sensor 10 is used downstream of the three-way catalytic converter 10 to correct the reference output for the theoretical air-fuel ratio of the oxygen concentration sensor 8 based on the deviation between the target value set for the theoretical air-fuel ratio and the output value. It

In such air-fuel ratio control, if the oxygen concentration sensor 8 deteriorates and the time constant becomes large, the calculation of the fuel amount fc supplied into the cylinder in the current combustion will be considerably delayed, and the target air-fuel ratio will be delayed. The fluctuation range of the air-fuel ratio centering on the fuel ratio becomes large.

By the way, as shown in FIG. 2, the exhaust gas purification performance of the three-way catalytic converter 10 is such that, as shown in FIG. 2, when the air-fuel ratio of the air-fuel mixture is near the stoichiometric air-fuel ratio, each harmful component can be purified well. You can Thus, when the time constant of the oxygen concentration sensor 8 is relatively small and the fluctuation range of the air-fuel ratio can be suppressed to a small value, NOx and HC are satisfactorily purified by setting the target air-fuel ratio to the theoretical air-fuel ratio (FIG. 2). Refer to the air-fuel ratio fluctuation range A). However, when the time constant becomes large and the fluctuation range of the air-fuel ratio becomes large as described above, if the target air-fuel ratio is the stoichiometric air-fuel ratio, the purification rate of HC is maintained relatively high, but the purification rate of NOx is reduced. The rate is extremely deteriorated (see the air-fuel ratio fluctuation range B in FIG. 2).

In order to solve this problem, the control device 20
Is adapted to change the target air-fuel ratio λt in the above-mentioned air-fuel ratio control according to the first flowchart shown in FIG. This first flow chart is executed when the engine is started and is repeated every predetermined time.

First, in step 101, the flag F
Is determined to be zero. The flag F is reset to 0 when the engine is stopped, and this determination is initially affirmed and the routine proceeds to step 102. In step 102, it is judged whether or not the oxygen concentration sensor 8 is activated by a general method, for example, whether the elapsed time after the engine is started is a predetermined time or more, or the cooling water temperature is a predetermined temperature or more. It If this judgment is denied, the process ends, but if affirmed, step 103
Proceed to.

At step 103, it is judged if the air-fuel ratio control during the normal operation is being executed. When this judgment is denied, it ends as it is,
When affirmative, it progresses to step 104. Step 10
4, the cooling water temperature THW is higher than THW1 and THW
It is judged whether it is lower than 2. This is the engine 1
Has warmed up and is in a normal state without overheating. When this determination is negative, the process ends as it is, but when the determination is affirmative, the process proceeds to step 105. In step 105, it is determined whether the integrated intake air amount QT after the determination in step 104 is affirmative is equal to or greater than a predetermined value QT1. When this judgment is denied, the process ends as it is, but when this judgment is affirmed, it means that the combustion of the engine 1 is sufficiently stable.
Proceed to 06.

In step 106, it is determined whether the present operating condition is a steady operating condition because the engine speed N and the intake air amount Q are constant. When this determination is negative, the process ends as it is, but when the determination is affirmative, the process proceeds to step 107. In step 107,
In calculation, the air-fuel ratio of the first cylinder and the third cylinder is the theoretical air-fuel ratio λ
The present intake air amount Q is maintained for a predetermined period so that the air-fuel ratio λ1 becomes richer and the air-fuel ratios of the second cylinder and the fourth cylinder become an air-fuel ratio λ2 leaner than the theoretical air-fuel ratio λ.
The fuel injection amount of each fuel injection valve 6 is determined based on the above, and the first fuel injection amount fixing control for fixing and injecting the fuel injection amount is executed.

Next, in step 108, step 1
It is determined whether or not the number of fuel injections n for each cylinder in the fuel injection amount fixed control of 07 is a predetermined number of times n1 or more. This determination is repeated until the determination is affirmative, and the process proceeds to step 109. In step 109, the output of the oxygen concentration sensor 8 is set in consideration of the first air-fuel ratio fluctuation waveform calculated based on the fuel injection amount after step 108 is affirmed and the time for the exhaust gas to reach the oxygen concentration sensor 8. The second air-fuel ratio fluctuation waveform calculated based on the above is compared. The first air-fuel ratio fluctuation waveform is, as shown in FIG. 4, the air-fuel ratio λ1 richer than the theoretical air-fuel ratio λ in the combustion of the first cylinder and the third cylinder that are continuously ignited, and the ignition is continuously performed after that. The air-fuel ratio λ2 is leaner than the theoretical air-fuel ratio λ in the combustion of the fourth cylinder and the second cylinder.

In the above-mentioned first air-fuel ratio fluctuation waveform, the fuel injection amount for each cylinder is fixed by the fuel injection amount fixing control in step 107 even if the above-mentioned deposit is present, and the injection is performed in each cylinder. Part of the injected fuel is absorbed by the deposit, but the same amount of fuel is discharged from the deposit, and the same amount of fuel as the injected fuel is supplied into the cylinder to form an air-fuel mixture. , Which corresponds to the actual fluctuation of the air-fuel ratio. Immediately after the start of fuel injection in step 107, the fuel injection amount may change with respect to the fuel injection amount before that, and at this time, the fuel absorption amount by the deposit and the fuel discharge amount from the deposit are not equal and the fuel is injected. Since the air-fuel ratio does not correspond to the fuel amount, the air-fuel ratio fluctuation at this time is not included in the first air-fuel ratio fluctuation waveform in step 108.

To compare the two waveforms in step 109, as is generally done, a vibration component of the two is extracted using a bandpass filter, and a first component for the vibration component of the first air-fuel ratio fluctuation waveform is extracted. 2 The cross spectrum value S of the vibration component of the air-fuel ratio fluctuation waveform is calculated. The cross spectrum value S is a value calculated based on the phase difference and the amplitude difference between the two.

Next, the process proceeds to step 110. Since the cross spectrum value S is a value that changes according to the engine speed N and the intake air amount Q for a predetermined period during which the fuel injection amount control of step 107 is being executed, the cross spectrum value S Normalization is performed. S ′ = A * S + B Here, A and B are variables determined from the first and second maps shown in FIGS. 5 and 6. In FIG. 5, A is based on an experiment so that the engine speed N gradually decreases from a low speed to a predetermined speed and then gradually increases with an increase in the rotation speed, and the intake air amount Q:
It is set so that the lower the rotation speed, the smaller the intake air amount, and the higher the rotation speed, the smaller the intake air amount. On the other hand, in FIG. 6, B is set based on an experiment so that the intake air amount is small and becomes large as the rotation speed is low.

Next, the routine proceeds to step 111, where the time constant K of the oxygen concentration sensor 8 is determined by the cross spectrum value S'normalized from the third map shown in FIG.
At 12, the flag F is set to 1. Next, the routine proceeds to step 113, where it is judged whether or not this time constant K is greater than or equal to a predetermined time constant K1. When this determination is denied, the time constant K of the oxygen concentration sensor 8 is relatively small, and the fluctuation range of the air-fuel ratio can be suppressed sufficiently small. The fuel ratio λt ends as the theoretical air-fuel ratio λ.

On the other hand, when the oxygen concentration sensor 8 deteriorates and the judgment in step 113 becomes affirmative, the routine proceeds to step 115, where it is determined from the fourth map shown in FIG. 8 based on the time constant K at this time. The target air-fuel ratio λt is set to the target air-fuel ratio λt ′. As can be seen from the fourth map shown in FIG. 8, the target air-fuel ratio λt is changed from the stoichiometric air-fuel ratio to the rich side as the time constant K increases due to the deterioration of the oxygen concentration sensor 8.

As shown in FIG. 2, the three-way catalytic converter 1
According to the characteristic of 0, when the air-fuel ratio of the air-fuel mixture shifts from the stoichiometric air-fuel ratio to the lean side, the purification rate of NOx sharply decreases, but even if it shifts to the rich side, the purification rate of HC does not decrease so much. Therefore, as described above, when the target air-fuel ratio λt is changed according to the time constant K of the oxygen concentration sensor 8, the maximum lean air-fuel ratio in the air-fuel ratio fluctuation range is always maintained near the stoichiometric air-fuel ratio. The NOx purification rate can be maintained high, while the maximum rich air-fuel ratio gradually deviates from the stoichiometric air-fuel ratio as the air-fuel ratio fluctuation range increases, but the HC purification rate does not decrease so much. In addition, both purification rates can be maintained relatively high (air-fuel ratio fluctuation range C in FIG. 2).
reference).

By the way, as described above, by changing various set values such as the coefficients P and R and various variables according to the current time constant K of the oxygen concentration sensor 8, the oxygen concentration sensor 8 can be changed.
It is possible to keep the variation range of the air-fuel ratio to a relatively small value even when the fuel cell deteriorates. However, in order to change such a plurality of setting values at the same time, a very complicated calculation is required. Therefore, instead of executing step 113 and the subsequent steps of the first flowchart, By executing the control of 1, the increase of the air-fuel ratio fluctuation range due to the increase of the time constant of the oxygen concentration sensor 8 is prevented.

The various set values in the air-fuel ratio control are changed depending on the output of the oxygen concentration sensor 8 when the oxygen constant sensor 8 is deteriorated and the time constant is relatively large. The value is set to a small value from the beginning by experiments or calculations.

In the air-fuel ratio control in which various set values are set in this way, the output of the oxygen concentration sensor 8 uses the value obtained by the following equation. Where _{V x = V X-1 +} (V i -V X-1) / C, the output value V _x is used for this control, V _X-1 is the output value as the previous control, V _i is the current The measured output value of the oxygen concentration sensor, V _X-1 is the output value of the oxygen concentration sensor measured last time, and C is the smoothed value.

The smoothing value C is determined from the fifth map shown in FIG. 9 based on the current time constant K of the oxygen concentration sensor 8 detected by the same flowchart (not shown) as before step 112 of the first flowchart. Is the value to be set.
In the fifth map, the predetermined time constant K2 is a time constant taken into consideration when setting various setting values, and when the actual time constant K becomes this value, the smoothed value C becomes 1 and is used for the control this time. The output value V _{x to be performed} and the output value V _i of the oxygen concentration sensor measured this time match.

As described above, the output value of the oxygen concentration sensor 8 used for control is always corrected to the output value when the time constant of the oxygen concentration sensor 8 becomes the predetermined time constant K2. In the air-fuel ratio control in which various set values are set in accordance with the predetermined time constant K2, even if the time constant becomes large due to deterioration of the oxygen concentration sensor 8, the fluctuation range of the air-fuel ratio is kept at least until it exceeds the predetermined time constant K2. The air-fuel ratio can be controlled to be almost constant.

[0034]

As described above, according to the air-fuel ratio control device of the first aspect, the air-fuel ratio is controlled and corrected based on the output of the oxygen concentration sensor so that the air-fuel ratio control means realizes the target air-fuel ratio. The larger the current time constant detected by the time constant detecting means, the larger the time constant of the oxygen concentration sensor, so that the target air-fuel ratio is changed from the stoichiometric air-fuel ratio to the rich side. However, since the center of fluctuation is on the rich side, the maximum lean air-fuel ratio of the air-fuel ratio fluctuation is maintained near the stoichiometric air-fuel ratio, and the NOx purification rate can be maintained high due to the characteristics of the three-way catalyst. At the same time, the maximum rich air-fuel ratio gradually deviates from the stoichiometric air-fuel ratio, but the purification rate of HC does not decrease so much due to the characteristics of the three-way catalyst, and even if the time constant of the oxygen concentration sensor becomes large, such a simple reduction occurs. By Do control can be maintained relatively high both purification rate.

Further, according to the air-fuel ratio control device of the second aspect, the various set values for the air-fuel ratio control are set by the set value determining means on the basis of predetermined time constants larger than the initial time constants of the oxygen concentration sensor, thereby achieving a good air-fuel ratio. Decided to achieve control,
The correction unit corrects the output of the oxygen concentration sensor to the output when the oxygen concentration sensor reaches a predetermined time constant based on the current time constant detected by the time constant detection unit, and various set values are set to oxygen. Even if the time constant of the oxygen concentration sensor changes, good air-fuel ratio control can always be achieved when the time constant of the oxygen concentration sensor changes without requiring complicated control such as changing according to the time constant of the concentration sensor. it can.

[Brief description of drawings]

FIG. 1 is a schematic diagram of an internal combustion engine equipped with an air-fuel ratio control device according to the present invention.

FIG. 2 is a graph showing the purification rate of the three-way catalyst with respect to the air-fuel ratio.

FIG. 3 is a flowchart for changing a target air-fuel ratio.

FIG. 4 is a diagram showing a first air-fuel ratio fluctuation waveform corresponding to an actual air-fuel ratio fluctuation.

FIG. 5: Variable A for normalizing cross spectrum values
It is a 1st map for determining.

FIG. 6 is a variable B for normalizing the cross spectrum value.
It is a 1st map for determining.

FIG. 7 is a third map for determining the time constant K of the oxygen concentration sensor.

FIG. 8 is a fourth map for determining a target air-fuel ratio.

FIG. 9 is a fourth map for determining a smoothed value.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Engine 3 ... Intake pipe 6 ... Fuel injection valve 7 ... Exhaust pipe 8 ... Oxygen concentration sensor 9 ... Three-way catalytic converter 10 ... Rear oxygen sensor 20 ... Control device

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁶ Identification code Internal reference number FI technical indication G01N 27/409

Claims (2)

[Claims] 1. An air-fuel ratio control means for controlling an air-fuel ratio based on an output of an oxygen concentration sensor arranged upstream of a three-way catalyst so that a target air-fuel ratio is realized, and a current time constant of the oxygen concentration sensor. A time constant detecting means for detecting and a target air-fuel ratio changing means for changing the target air-fuel ratio from the stoichiometric air-fuel ratio to a rich side as the current time constant detected by the time constant detecting means is larger, An air-fuel ratio control device comprising. 2. A time constant detecting means for detecting the current time constant of the oxygen concentration sensor, and a setting for determining various set values in the air-fuel ratio control based on a predetermined time constant larger than the initial time constant of the oxygen concentration sensor. A value determining means, and a correcting means for correcting the output of the oxygen concentration sensor to an output when the oxygen concentration sensor reaches the predetermined time constant, based on the current time constant detected by the time constant detecting means. Equipped with,
An air-fuel ratio control device for performing air-fuel ratio control using the output of the oxygen concentration sensor corrected by the correction means.