JP2002089318A - Exhaust emission control device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an exhaust emission control device intended to optimize cleaning efficiency of a three-way catalyst by varying a condition of an oxidizing atmosphere where an exhaust air fuel ratio is a lean air fuel ratio, and a condition of a reducing atmosphere where the exhaust air fuel ratio is a rich air fuel ratio so as to be well-balanced. SOLUTION: This exhaust emission control device comprises the three-way catalyst provided in an exhaust passage of an international combustion engine; an oxygen sensor provided at a downstream side of the three-way catalyst; an air fuel forced variation means capable of forcedly varying the air fuel ratio A/F with a given cycle and amplitude. The air fuel forced variation means regulate (S26, S30) to vary the air fuel ratio such that an output value of the oxygen sensor is a control target value S1 within the given range from a first given value (0.55 V) to a second given value (0.85 V).

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 apparatus, and more particularly to a technique for improving exhaust gas purifying efficiency in a three-way catalyst.

[0002]

[Related Background Art] A three-way catalyst used as an exhaust gas purifying catalyst can purify HC (hydrocarbon) and CO (carbon monoxide) with high efficiency in an oxidizing atmosphere (lean air-fuel ratio atmosphere), while reducing it. NOx can be purified with high efficiency in an atmosphere (rich air-fuel ratio atmosphere). For this reason, when the exhaust air-fuel ratio is largely biased toward either the lean air-fuel ratio or the rich air-fuel ratio, N
There is a problem that Ox or any one of HC and CO is exhausted without being sufficiently purified.

Therefore, conventionally, for example, Japanese Patent Publication No. 7-3379
As disclosed in Japanese Unexamined Patent Publication No. 3 (1993) -137 and Japanese Unexamined Patent Publication (Kokai) No. 9-137742, the air-fuel ratio (A / F), which is the ratio of fuel to air supplied to the internal combustion engine, is forcibly changed to thereby reduce the exhaust air-fuel ratio. It has been practiced to periodically fluctuate between a lean air-fuel ratio and a rich air-fuel ratio, thereby alternately generating an oxidizing atmosphere and a reducing atmosphere to purify NOx, HC, and CO without bias.

Further, a three-way catalyst in which a substance having an oxygen storage function (O ₂ storage function) such as ceria (Ce) is added to a carrier is known, and such a three-way catalyst is used together. For example, since O ₂ is stored in the catalyst in an oxidizing atmosphere, HC and CO are purified not only by NOx but also by the stored O ₂ even in a reducing atmosphere, whereby the purification efficiency can be further improved. is there.

[0005]

By the way, NOx is converted to C
In order to reduce with a reducing agent such as O, NOx needs to be adsorbed on the noble metal of the three-way catalyst, but in a reducing atmosphere state, the reducing agent such as CO is easily adsorbed on the noble metal,
As a result, there is a problem that adsorption of NOx to the noble metal is inhibited (this is called reduction poisoning). That is, if the reducing atmosphere state is continued for a long period of time, the reducing agent such as CO may cover the noble metal and may be discharged as it is without purifying NOx. In order to secure a larger amount of agent storage, it is better to maintain the reducing atmosphere for a long time.
There is a relationship that the length cannot be so long because the reducing agent such as O covers the noble metal.

On the other hand, in the three-way catalyst, a reducing agent such as CO adsorbed on a noble metal in a reducing atmosphere is oxidized by O ₂ stored in the catalyst.
It is preferable to increase the amount of O ₂ stored as much as possible. To this end, it is better to maintain the oxidizing atmosphere for a long time. On the other hand, in the oxidizing atmosphere, HC and CO are first purified by O ₂ and NOx cannot be sufficiently purified. Therefore, there is a relationship that it cannot actually be too long.

Furthermore, the ceria (Ce) is in the state of CeO ₂ promotes the water-gas reaction _{(CO + H 2 O → H} 2 + CO 2), H 2 produced by this (2NO + 2H ₂
→ N ₂ + 2H ₂ O) but is in the range purify NOx react with as inert water gas reaction since CO is often in a reducing atmosphere _{(CeO 2 + CO → Ce 2} O 3 + CO 2)
Occurs, and in this state, the water gas reaction is not promoted, so that the NOx purification efficiency may be reduced accordingly. In this regard, in order to minimize the inertness of the water gas reaction, it is preferable that the oxidizing atmosphere period is long and the reducing atmosphere period is not too long.

In view of the above, in order to optimize the purification efficiency of the three-way catalyst, it is necessary to determine whether the oxidizing atmosphere period in which the exhaust air-fuel ratio is lean and the reducing atmosphere period in which the rich air-fuel ratio is rich. The issue is how to make the modulation in a well-balanced manner. The present invention has been made to solve such a problem, and the purpose thereof is to:
It is an object of the present invention to provide an exhaust gas purifying apparatus that optimizes the purifying efficiency of a three-way catalyst by changing the oxidizing atmosphere state in which the exhaust air-fuel ratio is a lean air-fuel ratio and the reducing atmosphere state in which the rich air-fuel ratio is a rich air-fuel ratio. .

[0009]

In order to achieve the above object, according to the first aspect of the present invention, a three-way catalyst provided in an exhaust passage of an internal combustion engine and a three-way catalyst provided downstream of the three-way catalyst are provided. An oxygen sensor and air-fuel ratio forced variation means capable of forcibly varying an air-fuel ratio at a predetermined cycle and amplitude, wherein the air-fuel ratio forced variation means outputs an oxygen sensor having a first predetermined value or more and a second predetermined value or more.
It is characterized in that the air-fuel ratio is varied so as to be within a predetermined range equal to or less than a predetermined value.

Therefore, the air-fuel ratio is varied by the air-fuel ratio forced variation means so that the output value of the oxygen sensor falls within a predetermined range between the first predetermined value and the second predetermined value, thereby reducing the exhaust air-fuel ratio to the lean air-fuel ratio. The oxidation atmosphere state and the reduction atmosphere state with a rich air-fuel ratio can be modulated and varied in a well-balanced manner, thereby making it possible to optimize the purification efficiency of the three-way catalyst.

Here, claim 1 of the present invention has been made based on the following findings, and the operation of claim 1 will be described in detail below. When the air-fuel ratio of the internal combustion engine is changed by the air-fuel ratio forced change means, the exhaust air-fuel ratio periodically fluctuates between the lean air-fuel ratio and the rich air-fuel ratio. , CO
And NOx are purified respectively. At this time, the applicant examined the output value of an oxygen sensor provided downstream of the three-way catalyst, and found that the output value of the oxygen sensor was as shown in FIG. It has been confirmed that there is a certain relationship between and NOx purification efficiency.

That is, when the air-fuel ratio is periodically changed between a rich air-fuel ratio and a lean air-fuel ratio with a certain degree of modulation, FIG.
As indicated by the solid line, most of the output values of the oxygen sensor (for example, Denso 065500-2991 or an O ₂ sensor having an output voltage characteristic equivalent thereto) are mostly the first predetermined value (0.55 V (volt)). The second predetermined value (0.85V
(Volts)) It was confirmed that the NOx purification efficiency exhibited an optimum value (a state in which most of the NOx was purified) in the following fixed range.

Therefore, the output value of the oxygen sensor is equal to or more than the first predetermined value (0.55 V) and the second predetermined value (0.85 V).
V) If the air-fuel ratio is varied between the rich air-fuel ratio and the lean air-fuel ratio by setting the degree of modulation so as to fall within the predetermined range below, the NOx purification efficiency can be maintained in an optimal state. Becomes Therefore, by changing the air-fuel ratio so as to be within a predetermined range of not less than the first predetermined value and not more than the second predetermined value, it is possible to change the oxidizing atmosphere state and the reducing atmosphere state in a well-balanced manner, and to purify the three-way catalyst. It is possible to optimize the efficiency.

According to the second aspect of the present invention, a three-way catalyst provided in an exhaust passage of an internal combustion engine, an oxygen sensor provided downstream of the three-way catalyst, and a forcible air-fuel ratio at a predetermined cycle and amplitude. Air-fuel ratio compulsory fluctuation means that can vary the air-fuel ratio. The air-fuel ratio compulsory fluctuation means increases the exhaust gas transport delay so that the output value of the oxygen sensor falls within a predetermined range between a first predetermined value and a second predetermined value. The target value of the output value is set to a large value within the predetermined range to vary the air-fuel ratio.

Therefore, the air-fuel ratio is varied by the air-fuel ratio compulsory variation means so that the output value of the oxygen sensor falls within a predetermined range between the first predetermined value and the second predetermined value. By setting the target value of the output value to a large value within a predetermined range and changing the air-fuel ratio, even if the control responsiveness to the target value slows down due to a delay in exhaust transportation,
The oxidizing atmosphere state in which the exhaust air-fuel ratio is a lean air-fuel ratio and the reducing atmosphere state in which the rich air-fuel ratio is a rich air-fuel ratio are changed in a well-balanced manner, so that the purification efficiency of the three-way catalyst can be optimized without fail.

Here, claim 2 of the present invention has been made based on the following findings, and the operation of claim 2 will be described in detail below. According to the experiment conducted by the applicant, when the exhaust gas flow rate is low and the exhaust transportation delay is large, such as when the internal combustion engine rotation speed is low, the NOx purification efficiency is set to an optimum value (most of the NOx is smaller than when the transportation delay is small). It has been confirmed that a certain output range indicating the state of purification is expanded to the large side. That is, in FIG. 6, the NOx purification efficiency when the exhaust transportation delay is large is shown by a solid line, and when the exhaust transportation delay is small, the NOx purification efficiency is shown by a broken line. Becomes larger, the upper limit of the output value of the oxygen sensor at which the NOx purification efficiency shows the optimum value shifts to the larger side. Then, the maximum value of the upper limit becomes close to the second predetermined value (0.85 V).

On the other hand, when feedback control is performed by setting a control target value for the output value of the oxygen sensor,
When the exhaust gas transport delay increases and the response slows down, the variation from the control target value increases. That is, as shown by the solid line arrow in FIG. 6, if the exhaust transportation delay is small, the variation from the control target value is small, and if the exhaust transportation delay is large, the variation is large. When the variation from the control target value increases in this way, the output of the oxygen sensor temporarily deviates from the first predetermined value (0.55 V) when the exhaust transport delay is large while the control target value remains the same, NOx purification efficiency may be reduced.

However, in order to optimize the purification efficiency of the three-way catalyst, it is necessary to keep the range of variation from the control target value within a predetermined range. Therefore, taking into account the characteristic that the output range in which the NOx purification efficiency shows the optimum value shifts to the large side as described above, the target value of the output value of the oxygen sensor is relatively set to the first predetermined value (0 .55 V), it is considered better to shift the target value to a larger value as the exhaust gas transport delay increases.

For this reason, referring to FIG. 7, a map based on an experiment is shown. As shown in the map of FIG. 7, the target value of the output value is set within a predetermined range as the exhaust gas transport delay increases. If the air-fuel ratio is set to be large and the air-fuel ratio is changed, the response to the target value is slowed due to the delay in exhaust gas transport, and even if the variation from the control target value becomes large, as shown by the dashed arrow in FIG. Most of the output of the oxygen sensor always falls within a predetermined range equal to or higher than the first predetermined value (0.55 V), whereby the oxidizing atmosphere state and the reducing atmosphere state are changed in a well-balanced manner, and the three-way catalyst It is possible to optimize the purification efficiency.

According to a third aspect of the present invention, in the second aspect, the exhaust gas transport delay includes an exhaust flow velocity, an intake air amount, a vehicle speed, an oxygen sensor upstream exhaust system volume, an internal combustion engine rotation speed, a net average effective pressure, The detection is performed based on at least one index of an average effective pressure, a volume efficiency, an exhaust manifold pressure, an exhaust temperature, and an exhaust flow rate. Therefore, the exhaust gas transport delay can be easily and accurately detected by any easy means, thereby making it possible to reliably optimize the purification efficiency of the three-way catalyst.

According to a fourth aspect of the present invention, in the first to third aspects, the first predetermined value is 0.55 volt and the second predetermined value is 0.85 volt. I have. Therefore, the oxidizing atmosphere state and the reducing atmosphere state are balanced by changing the air-fuel ratio by the air-fuel ratio forced changing means so that most of the output value of the oxygen sensor falls within a predetermined range of 0.55 V to 0.85 V. It is possible to fluctuate well and fluctuate.

That is, based on the experimental results described above, an oxygen sensor (for example, DENSO 065500-2991)
Or an O ₂ sensor with equivalent output voltage characteristics)
, The first predetermined value is 0.55V, the second predetermined value is 0.85V, and therefore 0.55V to 0.85V.
Within the range of V, the oxidizing atmosphere state and the reducing atmosphere state can be modulated and varied in a well-balanced manner, and the purification efficiency of the three-way catalyst can be optimized.

[0023]

DESCRIPTION OF THE PREFERRED EMBODIMENTS One embodiment of the present invention will be described below with reference to the drawings. Referring to FIG. 1, there is shown a schematic configuration diagram of an exhaust gas purification device according to the present invention mounted on a vehicle. Hereinafter, the configuration of the exhaust gas purification device according to the present invention will be described with reference to FIG.

As shown in FIG.
For example, by switching the fuel injection mode, the fuel injection in the intake stroke (intake stroke injection) and the fuel injection in the compression stroke (compression stroke injection) 1
In-cylinder spark-ignition gasoline engine capable of performing the following is employed. The in-cylinder injection type engine 1 can be easily operated at a stoichiometric air-fuel ratio (stoichiometric ratio), at a rich air-fuel ratio (rich air-fuel ratio operation), or at a lean air-fuel ratio (lean air-fuel ratio operation). Is feasible.

As shown in FIG. 1, an electromagnetic fuel injection valve 6 is attached to a cylinder head 2 of an engine 1 together with a spark plug 4 for each cylinder, whereby fuel is directly injected into a combustion chamber. It is possible. An ignition coil 8 that outputs a high voltage is connected to the ignition plug 4. Further, a fuel supply device (not shown) having a fuel tank is connected to the fuel injection valve 6 via a fuel pipe 7. More specifically, the fuel supply device is provided with a low-pressure fuel pump and a high-pressure fuel pump, whereby the fuel in the fuel tank is supplied to the fuel injection valve 6 at a low fuel pressure or a high fuel pressure. Can be injected from the fuel injection valve 6 into the combustion chamber at a desired fuel pressure. At this time, the fuel injection amount is determined from the fuel discharge pressure Pinj of the high-pressure fuel pump and the valve opening time of the fuel injection valve 6, that is, the fuel injection time Tinj.

An intake port is formed in the cylinder head 2 in a substantially upright direction for each cylinder, and one end of an intake manifold 10 is connected to communicate with each intake port. An exhaust port is formed in the cylinder head 2 in a substantially horizontal direction for each cylinder, and one end of an exhaust manifold 12 is connected to communicate with each exhaust port.

Since the in-cylinder injection type engine 1 is already known, a detailed description of its configuration will be omitted. As shown in FIG.
Is provided with an electromagnetic throttle valve 14 for adjusting the intake air amount and a throttle position sensor (TPS) 16 for detecting the opening degree θth of the throttle valve 14.
Further, an air flow sensor 18 for measuring an intake air amount is provided upstream of the throttle valve 14. As the airflow sensor 18, a Karman vortex airflow sensor is used.

On the other hand, an exhaust pipe (exhaust passage) 20 is connected to the exhaust manifold 12, and a three-way catalyst 30 is interposed in the exhaust pipe 20 as an exhaust purification catalyst device. The three-way catalyst 30 is provided on a carrier as active noble metals such as copper (Cu), cobalt (Co), silver (Ag), and platinum (P).
t) and ceria (Ce) is added, and is configured as a three-way catalyst having an oxygen storage function (O ₂ storage function). That is, ceria (C
e) The property that when oxygen (O ₂ ) is adsorbed in an oxidizing atmosphere where the exhaust air-fuel ratio is a lean air-fuel ratio, the exhaust air-fuel ratio becomes a rich air-fuel ratio and maintains the state where O ₂ is adsorbed even in a reducing atmosphere. Accordingly, the three-way catalyst 30 has sufficient O ₂ on the surface of the carrier even in a reducing atmosphere state, so that HC (hydrocarbon) or CO (carbon monoxide)
Can be removed by oxidation. That is, the three-way catalyst 30 having the O ₂ storage function can not only purify HC and CO in an oxidizing atmosphere but also purify NOx not only in a reducing atmosphere but also in the reducing atmosphere by the occluded O _2. ,
CO can be purified well.

The exhaust pipe 20 is provided with a flow rate sensor 22 for measuring the exhaust flow rate. Further, an O ₂ sensor (oxygen sensor) 24 and a NOx sensor are provided downstream of the three-way catalyst 30.
A sensor 26 is provided. As the O ₂ sensor 24, here, an O ₂ sensor made by Denso (model number 0655) is used.
00-2991 or 234000-8181 or 2
34000-8211) or O ₂ sensor having a same output characteristics as it is used. NOx sensor 26 is NO
It detects the amount of x.

Also, an input / output device and a storage device (ROM, R
AM, nonvolatile RAM, etc.), central processing unit (CPU),
An ECU (electronic control unit) 40 having a timer counter and the like is provided, and the ECU 40 performs comprehensive control of the combustion control device including the engine 1. The input side of the ECU 40 includes the above-described TPS1.
6, air flow sensor 18, and various sensors such as flow sensors 22, O ₂ sensor 24 and the NOx sensor 26 is connected, the detection information from these sensors is input.

On the other hand, various output devices such as the above-described fuel injection valve 6 and ignition coil 8 are connected to the output side of the ECU 40. These various output devices are operated based on detection information from various sensors. The fuel injection amount, fuel injection timing, ignition timing, etc. are output, respectively,
An appropriate amount of fuel is injected from the fuel injection valve 6 at an appropriate timing, and spark ignition is performed by the spark plug 4 at an appropriate timing.

Hereinafter, the operation of the exhaust gas purifying apparatus according to the present invention will be described. In the exhaust gas purifying apparatus according to the present invention, the air-fuel ratio is forcibly and alternately changed between the rich air-fuel ratio and the lean air-fuel ratio by the ECU 40 in order to maximize the performance of the three-way catalyst 30. That is, here, as shown in FIG. 2, the air-fuel ratio (A / F)
Is set to a lean air-fuel ratio (for example, value 16) over a certain period (lean time), and then a rich air-fuel ratio (for example, value 1) for a certain period.
4), and the lean air-fuel ratio and the rich air-fuel ratio are periodically repeated. The modulation waveform is a square wave here, but may be a triangular wave.

Thus, when the exhaust air-fuel ratio is a lean air-fuel ratio, HC and CO are satisfactorily purified, O ₂ is stored by the O ₂ storage function of the three-way catalyst 30, and when the exhaust air-fuel ratio is a rich air-fuel ratio. NOx is successfully purified, and HC and CO are continuously purified by the stored O ₂ . Further, in the present invention, as described above, the oxygen sensor provided downstream of the catalyst, that is, the O ₂ sensor 24
It has been confirmed that there is a certain relationship between the output value of NO and the NOx purification efficiency, and the air-fuel ratio is modulated and controlled in a well-balanced manner in accordance with the output of the O ₂ sensor 24. The exhaust gas purifying efficiency of the source catalyst 30 is optimized (air-fuel ratio forced variation means).

Referring to FIG. 3, there is shown a flow chart of a control routine of the air-fuel ratio modulation control according to the output of the O ₂ sensor 24, that is, the A / F balance modulation control. A control procedure of the modulation control will be described. In step S10, first, it is determined whether or not the O ₂ sensor 24 is in the active state. Determination result escapes the routine when the O ₂ sensor 24 is false (No) is not in the active state, whereas, if the determination result is true (Yes), then the process proceeds to step S12.

In step S12, the modulation degree of the current air-fuel ratio, that is, the air-fuel ratio duty D which is the ratio of the lean air-fuel ratio time to the period T (lean time tl) is obtained from the following equation (1). It is determined whether or not it is smaller than a predetermined value D1 (D <D1). (Air-fuel ratio duty) D = (lean time) tl / (cycle) T (1) The value of the lean air-fuel ratio and the value of the rich air-fuel ratio are determined by the predetermined lean air-fuel ratio (for example, value 16) and the predetermined value. It is not necessary to fix each to a rich air-fuel ratio (for example, a value of 14), and an optimal value may be set according to each operating condition.

The cycle T may be a fixed value (for example, 1 second) or an operating state (for example, exhaust flow rate, intake flow rate, vehicle speed, catalyst temperature, exhaust pipe temperature, engine speed, net average effective value). Pressure, indicated mean effective pressure, volume efficiency, exhaust manifold pressure, at least one of cooling water temperature, and lubricating oil temperature).

Further, when the ignition timing and the like are set so that there is no difference in torque between the lean air-fuel ratio and the rich air-fuel ratio, the feeling is improved. As described above, the three-way catalyst 30
In order to control the air-fuel ratio in a well-balanced manner in order to optimize the exhaust gas purification efficiency, the output value (output S) of the O ₂ sensor 24 is changed from 0.55V (first predetermined value) to 0.85V (second predetermined value). The degree of modulation of the air-fuel ratio, that is, the duty D, may be set so as to be between (a predetermined value). The upper limit (D2) and the lower limit (D1) of the duty D that allow the output S to be within the desired range are set in advance by experiments.

That is, in step S12, the predetermined value D1, which is the determination value, is the duty D1 corresponding to the upper limit value 0.85V of the output S. However, the intake flow meter,
The actual lean air-fuel ratio or rich air-fuel ratio may be out of a desired range due to an error of the fuel injection valve or the like. At this time, even if the modulation degree of the air-fuel ratio is adjusted, the output S
Cannot be within the range of 0.55 V (first predetermined value) to 0.85 V (second predetermined value).

Therefore, in step S12, the duty D
Is smaller than the duty D1 corresponding to the upper limit of the output S of the present control, that is, it is determined whether the air-fuel ratio is deviated to the lean side so that the output S cannot be controlled within the desired range. I do. If the determination result of step S12 is true (Yes) and it is determined that the duty D is smaller than the predetermined value D1, the process proceeds to step S14.

In step S14, O ₂ output S of the sensor 24 it is determined whether or not smaller than the control target upper limit (S1 + ΔS1) (S < S1 + ΔS1). The control target value S1 is a value within the range of 0.55V and 0.85V, and the dead zone ΔS1 is, for example, 0.01V. By the way, as described above, when the feedback control is performed with the control target value determined, if the exhaust gas transport delay is large, the variation of the output S of the O ₂ sensor 24 with respect to the control target value increases. That is, the output S temporarily exceeds the control target upper limit value due to variation from the control target value of the output S.

Therefore, in practice, as shown in FIG. 6, the control target value S1 of the output S is adjusted so that most of the output S including this variation falls within the range of 0.55V to 0.85V. I am trying to set it. That is, the output range the NOx purification efficiency shows the optimum value, when narrow exhaust transport delay when the exhaust transport delay is small is large because it is expanded to the larger side, based on the map of FIG. 7, when the exhaust transport delay is less O ₂ The target value of the output S of the sensor 24 is set near the lower limit value of 0.55 V, and the target value is shifted and set to a larger value as the exhaust transportation delay increases.

Here, the exhaust transportation delay includes exhaust flow velocity, intake air amount, vehicle speed, exhaust system volume upstream of the oxygen sensor, internal combustion engine rotation speed, net average effective pressure, indicated average effective pressure, volume efficiency,
It can be detected by any of the exhaust manifold pressure, the exhaust temperature, and the exhaust flow rate.
The exhaust gas transport delay is detected based on the exhaust flow velocity information detected by (1). That is, it is determined that if the exhaust flow velocity is high, the exhaust transport delay is small, while if the exhaust flow velocity is low, the exhaust transport delay is large.

Although the instantaneous value of the O ₂ sensor 24 is used as the output S, an average value subjected to a smoothing process may be used. In this case, for example, an average value during the modulation cycle of the air-fuel ratio is used. If it is normal, the determination result of step S12 is true (Y
If es), the command value of the air-fuel ratio is excessively rich, and the output S must exceed the control target upper limit value (S1 + ΔS1). However, while the determination result of step S12 is true (Yes), the determination result of step S14 is true (Yes), and the output S is equal to the control target upper limit value (S1 + ΔS
If the value is smaller than 1), some control error occurs between the actual value and the measured value of the fuel injection amount and the intake air amount due to the abnormality of the intake flow meter, the fuel injection valve, and the like. On the other hand, it is considered that the actual air-fuel ratio is closer to the lean air-fuel ratio. Therefore, when the determination result of step S14 is true (Yes), the process proceeds to step S16.

In step S16, the command value of the air-fuel ratio is calculated by the following equation (2) so that the command value of the air-fuel ratio matches the actual air-fuel ratio.
To correct. That is, the predetermined lean air-fuel ratio (for example, value 16) and the predetermined rich air-fuel ratio (for example, value 14)
Is corrected to the rich side in order to match with the actual value. That is, upper / lower limit A / F enrichment correction is performed. Here (corrected A / F) (n) = ( corrected A / F) (n-1 ) + G1 ... (2), G1 is a correction gain, the output S and the control target value S1 of the O ₂ sensor 24 Increase or decrease according to the deviation.

Then, steps S12 to S1
Step 6 is repeatedly executed until the command value of the air-fuel ratio matches the actual air-fuel ratio. On the other hand, the determination result of step S14 is false (No), and the output S is the control target upper limit value (S1 + ΔS1).
When it is determined as described above, it is considered that the command value of the air-fuel ratio and the actual air-fuel ratio match. Therefore, in this case, the process proceeds to step S30, and the degree of modulation of the air-fuel ratio is adjusted so that the output S becomes the control target value S1. That is, by adjusting the degree of modulation of the air-fuel ratio, the average A / F shown in FIG. 2 is adjusted closer to the lean air-fuel ratio.

As a method of adjusting the modulation degree, a method of changing the air-fuel ratio duty D, a method of changing the supply degree of the oxidizing agent or the reducing agent (that is, the air-fuel ratio A / F), and the like can be considered. As shown in the following equation (3), a method of changing D is adopted to increase the time ratio of the lean air-fuel ratio, that is, to increase the air-fuel ratio duty D to make the average A / F lean and adjust the degree of modulation. .

[0047] D (n) = D (n -1) + G3 ... (3) Here, G3 is a correction gain, increases or decreases in accordance with the deviation between the output S and the control target value S1 of the O ₂ sensor 24. Incidentally, the output S of the O ₂ sensor 24 is non-linear with respect to the excess air ratio as shown by a solid line in FIG.
It changes suddenly in the vicinity, and it is difficult to control such a non-linear control target.

Therefore, here, the range of the control target value (0.55 V to 0.85 V) mainly depends on the excess air ratio of 1.
Since the area is smaller than 0, a pseudo output corresponding to the output S is set and the output S is linearized to widen the span in the output S direction, as shown by the broken line in FIG. , A pseudo target value is set in order to make the control easy and accurate.

Step S30 is repeated until the output S of the O ₂ sensor 24 reaches the target value, that is, until most of the output S including the variation falls within the range of 0.55V to 0.85V. Be executed. As a result, the oxidizing atmosphere state in which the exhaust air-fuel ratio is a lean air-fuel ratio and the reducing atmosphere state in which the rich air-fuel ratio is a rich air-fuel ratio can be modulated and varied in a well-balanced manner, and the purification efficiency of the three-way catalyst 30 can be optimized. It is planned.

If the result of the determination in step S12 is false (N
If it is determined in o) that the duty D is equal to or greater than the predetermined value D1, the process proceeds to step S18. In step S18, it is determined whether or not the duty D is larger than a predetermined value D2 (D> D2). The predetermined value D2, which is the discrimination value, is the duty D2 corresponding to the lower limit value 0.55V of the output S as described above. Here, it is determined whether the duty D is larger than the duty D2 corresponding to the lower limit of the output S of the present control. ,
That is, it is determined whether or not the air-fuel ratio may be shifted to the rich side so that the output S cannot be controlled within the desired range.

The result of the determination in step S18 is true (Yes).
If it is determined that the duty D is larger than the predetermined value D2, the process proceeds to step S20. In step S20, the output S of the O ₂ sensor 24 is set to the control target lower limit value (S1−Δ
It is determined whether it is larger than S1) (S> S1−ΔS1). If it is normal, the determination result of step S18 is true (Ye
If s), the command value of the air-fuel ratio is excessively lean, and the output S should be lower than the control target lower limit value (S1-ΔS1). However, while the determination result of step S18 is true (Yes), the determination result of step S20 is true (Yes), and the output S is the control target lower limit value (S1-ΔS1).
If it is larger than the above, it is considered that the actual air-fuel ratio is closer to the rich air-fuel ratio with respect to the air-fuel ratio command value, as described above. Therefore, the determination result of step S20 is true (Y
In the case of es), the process proceeds to step S22.

In step S22, the command value of the air-fuel ratio is calculated by the following equation (4) so that the command value of the air-fuel ratio matches the actual air-fuel ratio.
To correct. That is, the predetermined lean air-fuel ratio (for example, value 16) and the predetermined rich air-fuel ratio (for example, value 14)
Is corrected so as to match the command value with the actual value.
That is, upper / lower limit A / F leaning correction is performed. (Correction A / F) (n) = ( corrected A / F) (n-1 ) -G2 ... (4) Here, G2 is a correction gain, the output S and the control target value S1 of the O ₂ sensor 24 Increases or decreases according to the deviation of.

Then, similarly to the above steps S12 to S16, steps S18 to S22 are
It is repeatedly executed until the command value of the air-fuel ratio matches the actual air-fuel ratio. On the other hand, if the determination result of step S20 is false (No) and the output S is determined to be equal to or less than the control target lower limit value (S1−ΔS1), the command value of the air-fuel ratio is matched with the actual air-fuel ratio. It is thought that there is. Therefore, in this case, the process proceeds to step S26, in which the output S becomes the control target value S1.
The degree of modulation of the air-fuel ratio is adjusted so that That is, by adjusting the degree of modulation of the air-fuel ratio, the average A shown in FIG.
/ F is adjusted closer to the rich air-fuel ratio.

As in step S30, here, a method of changing the air-fuel ratio duty D is adopted, and as shown in the following equation (5), the time ratio of the rich air-fuel ratio is increased, that is, the air-fuel ratio duty D is reduced. Thereby, the average A / F is made rich to adjust the degree of modulation. D (n) = D (n -1) -G4 ... (5) Here, G4 is a correction gain, increases or decreases in accordance with the deviation between the output S and the control target value S1 of the O ₂ sensor 24.

This step S26 is repeated until the output S of the O ₂ sensor 24 reaches the target value, that is, until most of the output S including the variation falls within the range of 0.55V to 0.85V. Be executed. As a result, the oxidizing atmosphere state in which the exhaust air-fuel ratio is lean and the reducing atmosphere state in which the rich air-fuel ratio is rich can be modulated and changed in a well-balanced manner, and the purification efficiency of the three-way catalyst 30 can be optimized. Is achieved.

As described above, when the air-fuel ratio duty D is changed in step S26 or step S30,
The duty D falls within a range from a predetermined value D1 to a predetermined value D2. Therefore, in this case, step S
The judgment result of No. 18 is false (No), and then step S2
Proceed to 4. In step S24, it is determined whether or not the output S of the O ₂ sensor 24 is smaller than the control target lower limit value (S1−ΔS1) (S <S1−ΔS1).

Similarly, in step S28, it is determined whether or not the output S of the O ₂ sensor 24 is larger than the control target upper limit value (S1 + ΔS1) (S> S1 + ΔS1). Step S
24, the determination result of step S28 is true (Ye
In the case of s), as described above, the modulation degree of the air-fuel ratio is adjusted in step S26 or step S30, and as a result, the output S of the O ₂ sensor 24 is allowed to vary within the dead zone ΔS1, and the control target value is adjusted. Matches S1.

As a result, the output S of the O ₂ sensor 24 can be controlled so that the output S is always within the range of 0.55 V to 0.85 V, and the oxidizing atmosphere state and the reducing atmosphere state are always modulated in a well-balanced manner. The purification efficiency of the three-way catalyst 30 can be stably maintained in an optimum state. On the other hand, if both the determination results in step S24 and step S28 are false (No), in step S40, based on the NOx amount information detected by the NOx sensor 26, it is determined whether or not the NOx amount exceeds the predetermined amount X1. ((NOx amount)> X1) is determined.

When this step is executed, since the output S of the O ₂ sensor 24 is in the range of 0.55 V to 0.85 V, the NOx purification efficiency normally becomes the optimum value based on FIG. NOx should be almost completely purified by the three-way catalyst 30. Therefore, when it is determined that the NOx amount exceeds the predetermined amount X1, it is considered that the three-way catalyst 30 has deteriorated and the purification function has deteriorated. Therefore, here, the deterioration of the three-way catalyst 30 is determined by detecting the NOx amount downstream of the three-way catalyst 30.

If the result of the determination in step S40 is true (Yes), the process proceeds to step S42, where catalyst deterioration determination is performed. Specifically, the driver is notified of the abnormality by turning on an abnormality lamp or the like, and prompts repair. In the above-described embodiment, the upper / lower limit A / F and the average A / F are enriched or lean based on the equations (2) to (5). The control may be performed using at least one of the integral control and the differential control, or may be performed using a modern control theory.

In the above embodiment, the case where only the three-way catalyst 30 is provided in the exhaust passage has been described.
The present invention is also applicable to a case where a plurality of three-way catalysts are provided in the exhaust passage. For example, a normal three-way catalyst (rear catalyst) is arranged behind the exhaust pipe, a three-way catalyst (forward catalyst) is arranged near the engine 1, and an O ₂ sensor is provided immediately downstream of each three-way catalyst. In a situation where the rear catalyst cannot be sufficiently activated, such as at the time of a low temperature start, O downstream of the front catalyst is
To use a output value of the _second sensor, whereas after the rear catalyst is sufficiently active may be switched to use the output value of the rear catalyst downstream of the O ₂ sensor. The switching may be performed based on the operating state (for example, at least one of the cooling water temperature, the elapsed time after the start, the rear catalyst temperature, the front catalyst temperature, and the exhaust temperature), and the output value of the O ₂ sensor is completely changed. The output value of each O ₂ sensor may be weighted according to the operating state without switching to the above.

[0062] Further, controlling the air-fuel ratio by a catalyst downstream of the O ₂ sensor tends to delay with respect to changes in the engine air-fuel ratio by a catalyst such as the O ₂ sensor upstream increases. Therefore, if this delay becomes a problem, an exhaust gas sensor such as an O ₂ sensor or an A / F sensor may be further provided upstream of the catalyst, and correction may be performed based on the output.

[0063]

As described above in detail, according to the exhaust gas purifying apparatus of the first aspect of the present invention, the output value of the oxygen sensor is not less than the first predetermined value and not more than the second predetermined value by the forced air-fuel ratio varying means. By varying the air-fuel ratio so as to be within the predetermined range, the oxidizing atmosphere state in which the exhaust air-fuel ratio is lean and the reducing atmosphere state in which the rich air-fuel ratio is rich can be modulated and varied in a well-balanced manner. Thereby, the purification efficiency of the three-way catalyst can be optimized.

According to the exhaust gas purifying apparatus of the second aspect,
The air-fuel ratio is varied by the air-fuel ratio forced variation means so that the output value of the oxygen sensor falls within a predetermined range of not less than a first predetermined value and not more than a second predetermined value. Is set to a large value within a predetermined range to vary the air-fuel ratio, so that even if the control response to the target value slows due to a delay in exhaust transportation, the variation of the output value of the oxygen sensor with respect to the target value falls within the predetermined range. It is possible to change the oxidizing atmosphere state and the reducing atmosphere state in a well-balanced manner without deviating, and it is possible to reliably optimize the purification efficiency of the three-way catalyst.

According to the exhaust gas purifying apparatus of the third aspect,
According to the second aspect, the exhaust gas transport delay can be simply and accurately detected by any easy means, whereby the purification efficiency of the three-way catalyst can be surely optimized. Further, according to the exhaust gas purifying apparatus of the fourth aspect, in the first to third aspects, the oxygen sensor (for example, DENSO 065500-2991) is provided by the air-fuel ratio forced variation means.
Or an O ₂ sensor with equivalent output voltage characteristics)
By changing the air-fuel ratio so that the output value of the air-fuel ratio falls within a predetermined range of 0.55 V to 0.85 V, the oxidizing atmosphere state and the reducing atmosphere state can be modulated and changed in a well-balanced manner.

[Brief description of the drawings]

FIG. 1 is a schematic configuration diagram of an exhaust purification device according to the present invention mounted on a vehicle.

FIG. 2 is a diagram showing a modulation waveform of an air-fuel ratio (A / F).

FIG. 3 shows modulation control of an air-fuel ratio according to the output of an O ₂ sensor;
That is, it is a main part of a flowchart showing a control routine of A / F balance modulation control.

FIG. 4 is the remaining part of the flowchart showing the control routine of A / F balance modulation control following FIG. 3;

FIG. 5 is a diagram illustrating a method of linearizing the output S of the O ₂ sensor.

FIG. 6 shows an output of an O ₂ sensor provided downstream of a three-way catalyst and NOx.
It is an experimental result showing a relationship with purification efficiency.

FIG. 7 is a map showing a target value of an output value of an O ₂ sensor according to a delay in exhaust transportation.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Engine main body 4 Spark plug 6 Fuel injection valve 10 Intake manifold 12 Exhaust manifold 14 Throttle valve 16 Throttle position sensor (TPS) 18 Air flow sensor 20 Exhaust pipe (exhaust passage) 22 Flow rate sensor 24 O ₂ sensor (Oxygen sensor) 26 NOx sensor 30 three-way catalyst 40 ECU (electronic control unit)

 ──────────────────────────────────────────────────続 き Continued on the front page F-term (reference) HA08 HA36 HA37 3G301 HA01 HA04 HA18 JA21 JB10 LA03 MA01 NA01 NA03 NA04 NA05 ND01 ND18 ND41 NE13 NE15 NE17 NE19 NE23 PA01Z PA11Z PC02Z PD02Z PD11Z PD14Z PD16Z PE01Z PF01Z

Claims (4)

[Claims] 1. A three-way catalyst provided in an exhaust passage of an internal combustion engine, an oxygen sensor provided on a downstream side of the three-way catalyst, and an air forcibly changing an air-fuel ratio at a predetermined cycle and amplitude. Means for forcibly changing the fuel ratio.
An exhaust gas purification apparatus characterized in that the air-fuel ratio is changed so as to be within a predetermined range from a predetermined value to a second predetermined value. 2. A three-way catalyst provided in an exhaust passage of an internal combustion engine, an oxygen sensor provided on a downstream side of the three-way catalyst, and an air forcibly changing an air-fuel ratio at a predetermined cycle and amplitude. Means for forcibly changing the fuel ratio.
The exhaust gas is characterized in that the target value of the output value is set to a larger value within the predetermined range and the air-fuel ratio is varied as the exhaust gas transport delay is larger so as to be within a predetermined range of a predetermined value or more and a second predetermined value or less. Purification device. 3. The exhaust transportation delay includes an intake air amount, a vehicle speed, an oxygen sensor upstream exhaust system volume, an exhaust flow speed, an internal combustion engine rotation speed, a net average effective pressure, an indicated average effective pressure, a volume efficiency,
3. The exhaust gas purification apparatus according to claim 2, wherein the detection is performed based on at least one index of an exhaust manifold pressure, an exhaust temperature, and an exhaust flow rate. 4. The exhaust emission control device according to claim 1, wherein the first predetermined value is 0.55 volts, and the second predetermined value is 0.85 volts.