JPH05288106A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

PURPOSE:To judge degradation of a catalyst, to forcedly vary an air-fuel ratio correcting coefficient with a false signal at the time of non-degradation, and to prevent thermal degradation of an air-fuel ratio sensor while to operate the air-fuel ratio correcting coefficient based on inversion of a detected air-fuel ratio at the time of degradation, so as to prevent hunting of an air-fuel ratio. CONSTITUTION:An air-fuel ratio sensor 1 is arranged downstream of a catalytic converter interposed in an exhaust passage, and a control constant to be a base of an air-fuel ratio correcting coefficient is operated by a means 2 based on inversion of rich and lean in a detected air-fuel ratio. Moreover, the air-fuel ratio correcting coefficient is obtained by a first means 3 by synthesizing a specified false signal with a control constant while the air-fuel ratio correcting coefficient is obtained by a second means 4 based on inversion of the detected air-fuel ratio. Further, degradation of a catalyst is judged by a means 5 based on an inverted state of the detected air-fuel ratio while the first means 3 when not degraded, and the second means 4 when degraded are respectively chosen by a means 6. Fuel supplying quantity is corrected by a means 7 by using the air-fuel ratio correcting coefficient of chosen respective means 3, 4.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control device for an internal combustion engine, and more particularly to an air-fuel ratio control device provided with an air-fuel ratio sensor only on the downstream side of a catalytic converter.

[0002]

2. Description of the Related Art In a feedback type air-fuel ratio control device using an air-fuel ratio sensor, an air-fuel ratio sensor, for example, an O ₂ sensor is generally arranged upstream of a catalytic converter and its output signal is rich or lean. Based on the reversal, the fuel supply amount is feedback-controlled by, for example, proportional integral control.

In addition, air-fuel ratio sensors such as O ₂ sensors are provided upstream and downstream of the catalytic converter of the internal combustion engine, and air-fuel ratio feedback control is performed mainly by the output signal of the upstream O ₂ sensor. An air-fuel ratio control device that is executed and corrects the deviation of the air-fuel ratio of the entire control system based on the output signal of the downstream O ₂ sensor is disclosed in, for example, Japanese Patent Application Laid-Open No. 2-30915. ..

On the downstream side of the catalytic converter, the residual oxygen concentration changes very slowly due to the O ₂ storage capacity of the catalyst, and the rich / lean inversion cycle becomes very long. Proper feedback control cannot be performed with _two sensors alone.

[0005]

However, when the air-fuel ratio sensor is arranged on the upstream side of the catalytic converter as described above, deterioration due to exhaust heat is apt to occur and a change in characteristics over time becomes a problem. Further, if the air-fuel ratio sensors are arranged on both the upstream side and the downstream side, it goes without saying that the cost of parts increases.
The control system also becomes complicated.

Therefore, the present applicant previously calculated the air-fuel ratio correction coefficient as a composite signal of the control constant based on the sensor output and the pseudo signal of an appropriate period, and thus, the downstream side air-fuel ratio sensor alone can achieve good results. An air-fuel ratio control device capable of air-fuel ratio control has been proposed (Japanese Patent Application No. 3-154949).
issue).

Namely, as shown in FIG. 7, the downstream O ₂
The output signal Es of the sensor fluctuates in a relatively long cycle as shown in (a), and the control constant α ₁ calculated by proportional-plus-integral control from this also fluctuates gently as shown in (b). On the other hand, as shown in (c), a pseudo signal α ₂ having a constant cycle and amplitude is generated, and as a synthetic signal of the pseudo signal α ₂ and the control constant α ₁ , an air-fuel ratio correction as shown in (d) is performed. The coefficient α ₃ is calculated. By doing so, the air-fuel ratio on the inlet side of the catalytic converter fluctuates in a relatively short cycle which is the same as the pseudo signal α _2, and the conversion performance of the catalyst can be optimally maintained. Since the average correction amount is in line with the control constant α ₁ , the average air-fuel ratio can be maintained near the stoichiometric air-fuel ratio as in the case where the O ₂ sensor is arranged on the upstream side.

However, when the catalyst deteriorates in the catalytic converter, the reduction of O ₂ storage capability, the oxygen concentration is not changed so in the catalytic converter upstream side and a downstream side, as a result, O ₂ located downstream The output signal Es of the sensor is, as shown in FIG. 8A, the cycle of the air-fuel ratio change on the inlet side of the catalytic converter (that is, the cycle of the pseudo signal).
Inversion will be repeated at a cycle close to. As a result, the air-fuel ratio correction coefficient α ₃ combined with the pseudo signal α ₂ becomes (d)
As described above, hunting may occur and the air-fuel ratio deviation becomes large. This is not desirable, especially in situations where the catalytic converter is degraded.

[0009]

Therefore, according to the present invention,
Only the air-fuel ratio sensor on the downstream side of the catalytic converter is used, and optimal control is switched between when the catalyst is not deteriorated and when it is deteriorated. That is, the air-fuel ratio control system for an internal combustion engine according to the present invention, as shown in FIG. 1, is an air-fuel ratio sensor 1 arranged downstream of a catalytic converter interposed in an exhaust passage, and this air-fuel ratio sensor 1. The control constant calculation means 2 for calculating the control constant which is the basis of the air-fuel ratio correction coefficient based on the rich / lean inversion of the detected air-fuel ratio in the above, and a pseudo signal having a predetermined cycle and amplitude are combined with the control constant. First feedback control means 3 for obtaining an air-fuel ratio correction coefficient
And the rich air-fuel ratio detected by the air-fuel ratio sensor 1,
Second calculation of air-fuel ratio correction coefficient based on lean reversal
Feedback control means 4, a catalyst deterioration determination means 5 for determining catalyst deterioration based on the rich / lean inversion of the detected air-fuel ratio, and a first feedback control means 3 for determining non-deterioration. The fuel supply amount is corrected using the control mode selection means 6 for selecting the second feedback control means 4 when it is selected and determined to be deteriorated, and the air-fuel ratio correction coefficient of the selected feedback control means 3, 4. The correction means 7 is provided.

[0010]

Since the air-fuel ratio sensor located downstream of the catalytic converter reacts with the exhaust gas after passing through the catalytic converter,
If the catalyst is not deteriorated, the small air-fuel ratio fluctuation on the inlet side of the catalytic converter is not sensed by the O ₂ storage capacity of the catalyst. That is, the inversion of rich and lean is gently repeated. Further, when the catalyst deteriorates, it is also affected by small air-fuel ratio fluctuations on the inlet side. Therefore, the presence or absence of deterioration is determined by the catalyst deterioration determination means 5 based on the reversal period and the like.

When it is determined that the deterioration has not occurred, the first feedback control means 3 is selected. In this case, the air-fuel ratio correction coefficient is given as a composite signal of a pseudo signal having a predetermined cycle and amplitude and a control constant. The control constant is obtained by a proportional-integral control or the like based on rich or lean inversion of the detected air-fuel ratio. Therefore, the air-fuel ratio correction coefficient is
It fluctuates in the same cycle as the pseudo signal.

When it is judged that the deterioration has occurred, the second feedback control means 4 is selected. In this case, the air-fuel ratio correction coefficient is given by proportional-integral control based on rich or lean inversion of the detected air-fuel ratio. Since the catalyst is in a deteriorated state, the fluctuation cycle of the air-fuel ratio correction coefficient is relatively short.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the present invention will be described in detail below with reference to the drawings.

FIG. 2 is a structural explanatory view showing a mechanical structure of the air-fuel ratio control device according to the present invention, in which 11 is an internal combustion engine, 12 is an intake passage thereof, and 13 is an exhaust passage. An electromagnetic fuel injection valve 14 for supplying fuel to each intake port is arranged in the intake passage 12, and a throttle valve 15 is interposed. The opening of the throttle valve 15 is detected by a throttle opening sensor 16 including a potentiometer. Further, on the upstream side of the throttle valve 15, for example, a hot wire type air flow meter 17 for detecting the intake air amount is arranged.

A catalytic converter 18 using, for example, a three-way catalyst is interposed in the exhaust passage 13, and
An O ₂ sensor 19 is arranged as an air-fuel ratio sensor at a position downstream of the catalytic converter 18. This O ₂ sensor 1
No. 9 generates an electromotive force according to the residual oxygen concentration in the exhaust gas, and has a characteristic in which the electromotive force suddenly changes stepwise at the stoichiometric air-fuel ratio.

Reference numeral 20 denotes a water temperature sensor for detecting the cooling water temperature of the internal combustion engine 11, and reference numeral 21 denotes a crank angle sensor which is provided for detecting the engine speed and which issues a pulse signal at every predetermined crank angle.

The control unit 22 to which the detection signals of the various sensors described above are input uses a so-called microcomputer system, and includes a CPU 23 and a ROM 2.
4, the RAM 25, the I / O port 26, and the like. The control unit 22 controls the injection amount and injection timing of the fuel injection valve 14 based on the intake air amount detected by the air flow meter 17, the detection signal of the O ₂ sensor 19 and the like, and also controls the ignition timing of the engine and the like. Control.

Next, the operation of the above embodiment will be described. First, the outline of the air-fuel ratio control will be described. In this air-fuel ratio control, the basic pulse width Tp (basic injection amount) is calculated from the intake air amount Q detected by the air flow meter 17 and the engine speed N detected by the crank angle sensor 21 as follows: Tp = (Q / N) × k (However, k is a constant), and various increase corrections and feedback corrections are added to this to determine the actual drive pulse width Ti (injection amount) of the fuel injection valve 14. Specifically, The pulse width Ti is obtained by the equation.

Ti = Tp × COEF × α + Ts Here, COEF is various amount increase correction coefficients, and includes, for example, a water temperature increase correction coefficient based on the cooling water temperature and an air-fuel ratio correction coefficient at high speed and high load. Ts is a voltage correction coefficient added according to the battery voltage so as to compensate the dead time of the fuel injection valve 14.

Further, α is an air-fuel ratio correction coefficient which is calculated based on the detection signal of the O ₂ sensor 19, as will be described later. If the value of α is 1 or more, it goes to the rich side by 1 or less. If so, the air-fuel ratio is controlled to the lean side. As a result, in the feedback control region, the actual air-fuel ratio changes periodically and is maintained near the theoretical air-fuel ratio. Further, when the above-described high water temperature or high speed and high load is applied, the air-fuel ratio correction coefficient α is fixed to 1, and the open loop control is substantially performed.

Next, the calculation of the air-fuel ratio correction coefficient α will be described with reference to the flow charts of FIGS.

FIG. 3 shows a main flow chart which is repeatedly executed for each predetermined crank angle. In the portion shown in FIG. 3, the deterioration determination of the catalyst in the catalytic converter 18 is mainly performed, and the NORMFB (step 11, hereinafter abbreviated as S11 etc.) routine corresponding to the first feedback control means or the second feedback control means is equivalent. The SUBFB (S12) routine to be executed is selected. 4 and 5 show the NORMFB routine, and FIG. 6 shows the SUBFB routine.

First, in S1 of the flow chart of FIG.
Various signals such as the engine intake air amount Q, the engine speed N, the cooling water temperature TW, etc. are read, and it is determined in S2 whether or not the catalyst is in a predetermined operating state in which the catalyst deterioration can be determined. For example, the determination is not performed during a transition or during air-fuel ratio open loop control. S3
Then, the timer TIME delimiting a predetermined determination period is incremented, and the flag FL is set to 1 in S4. Based on the state of the flag FL, the number of inversions CT1 of the output of the O ₂ sensor 19 and the number of inversions CT2 of the pseudo signal α ₂ are counted in the NORMFB routine (S32).
-S34, S39-S41). And timer TIME
If the value of exceeds the predetermined value T2 (S5), the O ₂ sensor 19
Number of inversion of output CT1 and number of inversion of pseudo signal α ₂ CT
The ratio KK with 2 (= CT1 / CT2) is calculated (S7),
Moreover, it is determined whether or not the value of the KK exceeds the predetermined value KK1 (S8). At the same time, the timer TIME and the flag FL are reset (S6). That is, the above-described inversion times CT1 and CT2 indicate the number of times detected during the predetermined period T2.

If the catalyst is not deteriorated, the rich / lean inversion cycle of the output Es of the O ₂ sensor 19 is long due to the O ₂ storage capacity of the catalyst, as shown in FIG. The reverse rotation CT1 inside is small. Therefore, the above-mentioned ratio KK is usually a small value of KK1 or less. On the other hand, when the catalyst is considerably deteriorated, as shown in FIG. 8A, the output Es of the O ₂ sensor 19 changes due to a slight change in the air-fuel ratio on the inlet side of the catalytic converter 18, The reverse rotation CT1 becomes large. Therefore, the above-mentioned ratio KK exceeds KK1. In this S8, KK
When it is determined that> KK1, the counter P is incremented (S9).

In S10, it is determined whether the value of the counter P exceeds a predetermined value P1, and if it is less than P1, the process proceeds to S11 and NORMF corresponding to the first feedback control means.
B is selected (S11). Since the counter P is initialized to 0 when the engine is started, NORMFB is always executed until the catalyst deterioration is detected. And
When the counter P exceeds P1, it is determined that the catalyst has deteriorated, and the SU corresponding to the second feedback control means.
BFB is selected (S12). Further, once P> P1, P = P2 is fixed (however, P2> P1), and SUBFB is continued until the engine is stopped (S13).

Next, based on FIGS. 4 and 5, NO in S11.
The RMFB routine will be described.

First, various signals such as the engine intake air amount Q, the engine speed N, the cooling water temperature TW are read in S21, and S22
Then, it is determined whether or not a predetermined air-fuel ratio feedback control condition is satisfied. If YES here, proceed to S23,
The output voltage Es of the O ₂ sensor 19 is A / D converted and read. In S24, the output voltage Es is compared with a predetermined slice level VREF (corresponding to a substantially stoichiometric air-fuel ratio) to make a rich / lean determination. That is, if Es ≦ VREF, it means lean and the process proceeds to S25. If E> VREF, it is rich, and the process proceeds to S28. If it is lean, then in S25, the previous rich or lean state is determined based on the flag C1. This flag C1 is C
1 = 0 indicates the previous lean state, and C1 = 1
Indicates that it was in the rich state last time. C1 = in S25
1, that is, the previous rich state means that the detected air-fuel ratio is inverted from the rich state to the lean state, the flag C1 is set to 0 (S26), and the previous control constant α ₁ is set to the predetermined value in S35. Proportional P is added. At the same time, the above-mentioned flag FL is determined (S32), and if the flag FL is 1, the counter CT1 is incremented to count the number of inversions (S33). If the flag FL is 0, the counter CT1 is reset (S34).
Further, if C1 = 0 in S25, that is, if the lean state was the last time, it means that the lean state continues, and S2
In step 7, a predetermined integral I is added to the previous control constant α ₁ .
On the other hand, if it is rich in S24, the previous rich and lean states are similarly determined in S28, and if the lean state is reversed to the rich state, the flag C1 is set to 1 (S29) and the previous A predetermined proportional amount P is subtracted from the control constant α ₁ (S30). If the rich state continues, a predetermined integral I is subtracted from the previous control constant α _{1 in} S31. By the above processing, O
₍₂₎ Proportional-integral control based on the output signal of the sensor 19 is realized, and the control constant α ₁ changes as shown in FIG. 7B.

Next, in S36 of FIG. 5, a predetermined value T is added to the value of the previous timer T1. This timer T1
It is reset every time the pseudo signal α ₂ shown in FIG. 7C rises (S42). Therefore, the elapsed time from here is counted. Then, in S17 and S18, the value of the timer T1 is set to the predetermined values TL1 and TL2, respectively.
(However, compared with TL2 ≒ 2 · TL1), T1 is TL1
If the following is true, the value of the pseudo signal α ₂ is set to α ₂ = 1 in S45. If T1 is between TL1 and TL2, the value of the pseudo signal α ₂ is set to α ₂ = 0 in S44. When T1 reaches TL2, the timer T1 is reset as described above (S4
2). At the same time, the flag FL described above is determined (S3
9) If the flag FL is 1, the counter CT2 is incremented to count the number of inversions of the pseudo signal α ₂ (S40). If the flag FL is 0, the counter CT2 is reset (S41).

By the processing of S36 and thereafter, the pseudo signal α ₂ shown in FIG. 7C is generated. The pseudo signal α ₂ becomes a pulse signal having a constant amplitude of 0 to 1 and a constant period (TL2), and TL1 corresponds to the pulse width thereof.
The period TL2 is set so that the conversion performance of the catalytic converter 18 is the best, and is, for example, 2
Although given in the order of Hz, more specifically, both TL1 and TL2 are set by searching from a map using the intake air amount Q, the engine speed N, and the cooling water temperature TW as parameters.

Then, in S43, the control constant α ₁ and the value of the pseudo signal α ₂ are combined as shown in the following equation to obtain the air-fuel ratio correction coefficient α ₃ , which is calculated as described above. Used as.

Α ₃ = α ₁ + K ₁ (α ₂ −K ₂ ) Here, K 1 and K 2 are constants, which are also set by searching from a map using the intake air amount Q, the engine speed N, and the cooling water temperature TW as parameters. To be done. The air-fuel ratio correction coefficient α ₃ thus obtained, as shown in FIG. 7D, changes relatively finely with the period of the pseudo signal α ₂ , and its average change tendency Is along the control constant α ₁ as shown by the broken line. Therefore, a feedback system based on the detection signal of the O ₂ sensor 19 is formed, and the average air-fuel ratio is maintained with good responsiveness in the vicinity of substantially the theoretical air-fuel ratio. The air-fuel ratio on the inlet side of the catalytic converter 18 fluctuates in an appropriate cycle regardless of the rich / lean inversion cycle of the O ₂ sensor 19, and the conversion performance in the catalytic converter 18 becomes good.

Next, S of S12 selected when the catalyst deteriorates.
The UBFB routine will be described with reference to FIG.

This SUBFB routine basically performs the proportional-integral control similar to the calculation of the control constant α ₁ described above. First, at S51, the engine intake air amount Q, the engine speed N, and the cooling water temperature. Read various signals such as TW, S5
At 2, it is determined whether or not a predetermined air-fuel ratio feedback control condition is satisfied. If YES here, the flow proceeds to S53, where the output voltage Es of the O ₂ sensor 19 is A / D converted and read. In S54, this output voltage Es is changed to S24 described above.
Similarly to, the slice level VREF is compared and the rich / lean determination is made. If Es ≦ VREF, it means lean and the process proceeds to S55. If Es> VREF, it is rich and the process proceeds to S59. If it is lean, then in S55, the previous rich or lean state is determined based on the flag C2. This flag C2 indicates that C2 = 0 was the lean state last time, and C2 = 1 indicates the previous rich state. If C2 = 1 in S55, that is, the previous rich state, the detected air-fuel ratio is inverted from the rich state to the lean state, the flag C2 is set to 0 (S56), and the previous air-fuel ratio correction is performed in S57. A predetermined proportional amount P ′ is added to the coefficient α ₄ . In addition, C in S55
When 2 = 0, that is, when the lean state is present also in the previous time, it means that the lean state continues, and in S58, a predetermined integral I'is added to the previous air-fuel ratio correction coefficient α ₄ . on the other hand,
If it is rich in S54, the previous rich and lean states are similarly determined in S59, and when the lean state is reversed to the rich state, the flag C2 is set to 1 (S
60), and a predetermined proportional amount P ′ is subtracted from the previous air-fuel ratio correction coefficient α ₄ (S61). If the rich state continues, a predetermined integral I'is subtracted from the previous air-fuel ratio correction coefficient α ₄ in S62. Then, in this SUBFB, the value of α ₄ obtained as described above is used as it is as the air-fuel ratio correction coefficient α.

Therefore, when the catalyst is deteriorated, the air-fuel ratio correction coefficient α ₄ (= α) is as shown in FIG. 8 (e) with respect to the detection signal of the O ₂ sensor 19 output as shown in FIG. 8 (a). The fuel injection amount Tp is corrected by the given air-fuel ratio correction coefficient α ₄ . Therefore, hunting due to interference with the pseudo signal α ₂ as shown in (d) does not occur, and stable control with a small deviation can be performed. In this state, the fluctuation of the air-fuel ratio on the inlet side of the catalytic converter 18 is O ₂
Since it affects the sensor 19, the fluctuation cycle of the air-fuel ratio correction coefficient α ₄ naturally shortens, and the system responsiveness is not deteriorated and the conversion performance is not deteriorated due to an excessively long cycle.

[0035]

As is apparent from the above description, according to the air-fuel ratio control system for an internal combustion engine according to the present invention, when it is determined that the catalyst is not deteriorated based on the deterioration determination of the catalyst, the air-fuel ratio is determined by the cycle of the pseudo signal. Since the correction coefficient is forcibly changed,
Feedback control with high catalyst conversion performance can be realized only by the air-fuel ratio sensor downstream of the catalytic converter, and the problem of thermal deterioration of the air-fuel ratio sensor can be avoided. When the catalyst deteriorates, the synthesis with the pseudo signal is stopped, and the air-fuel ratio correction coefficient is directly calculated based on the rich / lean inversion of the detected air-fuel ratio in the air-fuel ratio sensor on the downstream side of the catalytic converter. Hunting is prevented, and an increase in air-fuel ratio deviation can be avoided.

[Brief description of drawings]

FIG. 1 is a claim correspondence diagram showing a configuration of the present invention.

FIG. 2 is a structural explanatory view showing an embodiment of an air-fuel ratio control device according to the present invention.

FIG. 3 is a flowchart showing a flow of calculation processing of an air-fuel ratio correction coefficient α.

FIG. 4 is a flowchart showing the first half of the NORMFB routine.

FIG. 5 is a flowchart showing the latter half of the NORMFB routine.

FIG. 6 is a flowchart showing a SUBFB routine.

FIG. 7 is a waveform diagram showing, in comparison, each signal when the catalyst is not deteriorated.

FIG. 8 is a waveform diagram showing, in comparison, each signal when the catalyst is deteriorated.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Air-fuel ratio sensor 2 ... Control constant calculation means 3 ... 1st feedback control means 4 ... 2nd feedback control means 5 ... Catalyst deterioration determination means 6 ... Control mode selection means 7 ... Correction means

Claims (1)

[Claims] 1. A base of an air-fuel ratio correction coefficient based on an air-fuel ratio sensor disposed downstream of a catalytic converter interposed in an exhaust passage and rich / lean inversion of a detected air-fuel ratio in this air-fuel ratio sensor. Control constant calculating means for calculating a control constant, first feedback control means for synthesizing a pseudo signal having a predetermined period and amplitude with the control constant to obtain an air-fuel ratio correction coefficient, and detection by the air-fuel ratio sensor. Second feedback control means for calculating an air-fuel ratio correction coefficient based on rich / lean inversion of the air-fuel ratio, and catalyst deterioration determination means for determining catalyst deterioration based on the manner of the detected air-fuel ratio rich / lean inversion. When the non-deterioration is determined, the first feedback control means is selected,
A control mode selection unit that selects the second feedback control unit when it is determined to be deteriorated and a correction unit that corrects the fuel supply amount using the air-fuel ratio correction coefficient of the selected feedback control unit are provided. An air-fuel ratio control device for an internal combustion engine, which is characterized.