JP2003206784A - Air-fuel ratio control device of engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To improve the reliability of correction of air-fuel ratio control in an engine which performs the air-fuel ratio control based on the output of an oxygen sensor at the upstream of a three-way catalyst, and corrects the air-fuel ratio control based on the output of the oxygen sensor at the downstream. <P>SOLUTION: A control unit 10 detects the degree of degradation of the three- way catalyst 7 from the output of a sensor 8 at the upstream and a sensor 9 at the downstream, and the larger the degree of degradation is, the more the correction to the feedback control of the air-fuel ratio is reduced. By not unifying the correction, but changing the correction according to the degree of degradation of the three-way catalyst 7, the correction is neither excessive nor insufficient, and the catalyst 7 is neither in a oxygen-depleted condition nor in an oxygen-saturated condition, and the oxygen of the balanced optimum quantity is occluded by the catalyst 7 to achieve the optimum control of the exhaust gas. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an engine air-fuel ratio control system. More specifically, the air-fuel ratio control is performed using the oxygen sensor installed upstream of the exhaust gas purifying catalyst arranged in the exhaust system, and the air-fuel ratio control is corrected using the oxygen sensor installed downstream. The engine configured as described above belongs to the technical field of improving the reliability of correction of the air-fuel ratio control.

[0002]

2. Description of the Related Art For example, OBD-2 (On-Board Diagnostic Sys
According to tems Stage 2: Vehicle exhaust gas diagnostic device regulation second stage), it is obligatory to monitor the exhaust gas control status of an automobile with an in-vehicle computer. The deterioration determination of the oxygen sensor and the exhaust gas purifying catalyst installed in the exhaust system of the engine is listed as a regulation item. As a technique for determining catalyst deterioration, a technique that utilizes the number of times of output reversal between rich and lean of two oxygen sensors installed on the upstream side and the downstream side of the catalyst is widely known. For example, when the catalyst is normal and the oxygen storage capacity is large, the output reversal count of the downstream oxygen sensor approaches zero, so the value of the ratio of the output reversal count of the upstream oxygen sensor to the output reversal count of the downstream oxygen sensor is Grows (infinity). vice versa,
When the catalyst deteriorates and the oxygen storage capacity is small, the number of output reversals of the downstream oxygen sensor approaches the number of output reversals of the upstream oxygen sensor, so the value of the ratio becomes small (close to 1). Therefore, it can be determined that the catalyst has deteriorated when the value of the above ratio becomes equal to or less than the predetermined value.

A three-way catalyst generally used as an exhaust gas purifying catalyst is an air-fuel ratio (combustion) capable of efficiently purifying all three elements of hydrocarbon (HC), carbon monoxide (CO), and nitrogen oxide (NOx). Since the range (window) of the air-fuel ratio of the air-fuel mixture supplied to the chamber is limited to a narrow range around the theoretical air-fuel ratio (A / F = 14.7), the air-fuel ratio should be close to the theoretical air-fuel ratio. Feedback control is performed. This air-fuel ratio feedback control is disclosed in JP-A-6-2806.
As disclosed in Japanese Patent Laid-Open No. 49-83830 and Japanese Patent Laid-Open No. 8-303280, the residual oxygen concentration in exhaust gas before passing through the catalyst is detected by using the above-mentioned upstream oxygen sensor, and the output of the sensor is theoretically empty. When the state is leaner than the fuel ratio, the air-fuel ratio is corrected to the rich side by increasing the fuel supply amount or reducing the intake air amount, while the sensor output is richer than the theoretical air-fuel ratio. When is shown, the air-fuel ratio is corrected to the lean side by reducing the fuel supply amount or increasing the intake air amount.

However, the catalyst gradually deteriorates according to the traveling distance, the use conditions of the user, the use environment, etc. (heat loss or poisoning due to sulfur or phosphorus components contained in the fuel), and as a result, Since the optimum purification air-fuel ratio (window) fluctuates, even if feedback control of the air-fuel ratio is performed as described above, the air-fuel ratio gradually deviates from the window, and optimal purification by the three-way catalyst cannot be realized. For example, when the air-fuel ratio shifts from the window to the rich side, the emissions of hydrocarbons and carbon monoxide increase, and conversely, when the air-fuel ratio shifts to the lean side, the emissions of nitrogen oxides increase.

Therefore, when the residual oxygen concentration in the exhaust gas after passing through the catalyst is detected by using the above downstream oxygen sensor and the output of the sensor is biased to the lean side of the stoichiometric air-fuel ratio (oxygen Is leaking beyond the oxygen storage capacity of the catalyst), the air-fuel ratio control is corrected to the rich side,
If the sensor output deviates to the rich side of the stoichiometric air-fuel ratio (when oxygen does not leak from the catalyst) while promoting oxygen consumption and canceling the lean atmosphere, the above air-fuel ratio It is known that the control is corrected to the lean side to promote the occlusion of oxygen and cancel the rich atmosphere.

The correction of the air-fuel ratio control can be achieved, for example, by correcting the output of the upstream oxygen sensor itself or by correcting the feedback correction amount Cfb used when setting the fuel injection amount. You can

[0007]

However, conventionally, since the correction amount for the above air-fuel ratio control is uniform, the following problems have occurred. That is, the greater the degree of deterioration of the catalyst, the smaller the oxygen storage capacity, and conversely, the smaller the degree of deterioration of the catalyst, the larger the oxygen storage capacity.For example, oxygen leaks out beyond the oxygen storage capacity of the catalyst, and the downstream side For example, when the output of the oxygen sensor is biased to the lean side and the air-fuel ratio control is corrected to the rich side, when the deterioration degree of the catalyst is large, the oxygen storage capacity is relatively small and the correction is excessive. The oxygen tends to be slightly released, and the oxygen stored in the catalyst is excessively released, so that the oxygen is hardly stored in the catalyst or the oxygen is not stored at all. In such an oxygen depleted state,
When the oxygen storage effect of the catalyst is not utilized and the air-fuel ratio is switched to the rich state by feedback control next time, the emission amount of hydrocarbons and carbon monoxide immediately increases.

On the contrary, when the degree of deterioration of the catalyst is small, the oxygen storage capacity is relatively large, so that the correction tends to be inadequate, and the oxygen stored in the catalyst is released only in a small proportion. The saturation condition is not resolved. In this state, when the lean state continues, oxygen leaks again,
Immediately the emission of nitrogen oxides increases. After all, ideally, it is preferable that the catalyst always stores the optimum amount of oxygen in a sufficient amount.

Contrary to the above, such a problem is caused when oxygen does not leak from the catalyst, the output of the downstream oxygen sensor is biased to the rich side, and the air-fuel ratio control is corrected to the lean side. Can occur as well. That is, when the degree of deterioration of the catalyst is large, as a result of overshooting, the catalyst is in an oxygen saturated state, and conversely, when the degree of deterioration of the catalyst is small,
As a result of lack of correction, the oxygen depletion state of the catalyst is not resolved.

The present invention has been made in view of the above circumstances, and performs air-fuel ratio control based on the output of the upstream oxygen sensor and corrects the air-fuel ratio control based on the output of the downstream oxygen sensor. In the engine control system configured as described above, it is an object to improve the reliability of the correction of the air-fuel ratio control so that the catalyst always stores the optimum amount of oxygen.

[0011]

That is, the invention as set forth in claim 1 is to provide an air-fuel mixture supplied to a combustion chamber based on an output of an oxygen sensor upstream of a three-way catalyst arranged in an exhaust system of an engine. Of the engine having an air-fuel ratio control means for controlling the air-fuel ratio of the above to the stoichiometric air-fuel ratio, and an air-fuel ratio control correction means for correcting the air-fuel ratio control by the control means based on the output of the oxygen sensor downstream of the three-way catalyst An air-fuel ratio control device, comprising: a correction amount changing device for changing a correction amount by the air-fuel ratio control correcting device; and a deterioration degree detecting device for detecting a deterioration degree of a three-way catalyst. The correction amount is smaller as the deterioration degree of the three-way catalyst detected by the deterioration degree detecting means is larger.

According to the present invention, the correction amount for the air-fuel ratio control is not uniform, but is changed according to the degree of deterioration of the three-way catalyst. Specifically, the larger the degree of catalyst deterioration, the smaller the correction amount, and the smaller the degree of catalyst deterioration, the larger the correction amount. When the degree of deterioration of the catalyst is large and the oxygen storage capacity is small, the correction from lean to rich or from rich to lean seems to be suppressed, so the correction does not go too far and the catalyst is in an oxygen depleted state (from lean to rich). Case) or oxygen saturation state (from rich to lean), the optimum amount of oxygen is stored without excess or deficiency, and optimum purification by the three-way catalyst is achieved. On the contrary, when the degree of deterioration of the catalyst is small and the oxygen storage capacity is large, the correction from lean to rich or from rich to lean is likely to be accelerated, so that the correction is not insufficient and the catalyst is still in an oxygen depleted state ( An optimal amount of oxygen is stored without excess or deficiency without achieving the rich to lean case or the oxygen saturation state (from the lean to rich case), and the optimal purification by the three-way catalyst is achieved.

Next, the invention described in claim 2 is, similar to the invention described in claim 1, air-fuel ratio control means for performing air-fuel ratio control based on the output of the oxygen sensor upstream of the three-way catalyst,
An air-fuel ratio control device for an engine, comprising: an air-fuel ratio control correction means for correcting the air-fuel ratio control based on the output of an oxygen sensor downstream of a three-way catalyst, wherein the correction amount by the air-fuel ratio control correction means is changed. A correction amount changing means is provided, and when the output of the downstream side oxygen sensor is biased to the rich side of the stoichiometric air-fuel ratio, the correction amount is changed as compared with the lean side. It is characterized in that it is configured to be small.

Like the invention described in claim 1, the present invention intends to improve the reliability of the correction of the air-fuel ratio control by changing the correction amount for the air-fuel ratio control in a non-uniform manner. , The downstream oxygen sensor output is biased to the rich side, the correction amount when correcting the air-fuel ratio control to the lean side is made relatively small, and the downstream oxygen sensor output is biased to the lean side. The correction amount when the air-fuel ratio control is corrected to the rich side is relatively increased.

That is, this type of oxygen sensor is generally called a λO2 sensor or the like, and the output voltage greatly changes depending on whether the air-fuel ratio is richer or leaner than the theoretical air-fuel ratio,
When rich, the output voltage becomes high (for example, about 1V),
The output voltage is low (for example, approximately 0 V) when lean. Therefore, when the downstream oxygen sensor output is biased to the lean side, it is apparent that oxygen leaks out beyond the oxygen storage capacity of the catalyst, so this may correct the air-fuel ratio to the rich side. It is in a state. Moreover,
This is a state in which rich correction may be performed with a slight tendency to accelerate.

On the other hand, when the output of the downstream oxygen sensor is biased to the rich side, it means that oxygen does not leak from the catalyst, and how much oxygen is stored in the catalyst. I don't know if. Oxygen is still stored in the catalyst, and the stored oxygen is HC, C
As a result of being consumed for the oxidation of O, if the oxygen does not leak from the catalyst, the exhaust purification performance is kept in a good state. In this case, it is not necessary to make the correction on the lean side. Alternatively, even if this is done, lean correction may be performed to a slight extent.

However, if oxygen is not leaking from the catalyst as a result of exhaustion of all the stored oxygen and depletion of the catalyst, this is just a rich state, and HC, CO emissions are increasing.
In this case, lean correction needs to be positive. Moreover, it is necessary to make a lean correction to facilitate acceleration.

However, as described above, when the output of the downstream oxygen sensor is biased to the rich side, it is not known which of these states the catalyst is in. Therefore, as a whole, it is appropriate to reduce the correction amount and suppress the correction compared with when the output of the downstream oxygen sensor is biased to the lean side. On the other hand, when the output of the downstream oxygen sensor is biased to the lean side, the correction amount is increased and the correction is likely to be accelerated, so the lean state is resolved within a short time and the NOx emission amount is suppressed. To be done.

In any case, rational correction of the air-fuel ratio control is made according to the situation, and as a result, the catalyst does not become in an oxygen-depleted state or an oxygen-saturated state, and the optimum amount is sufficient. Oxygen is occluded and optimum purification by the three-way catalyst is achieved.

Next, the invention described in claim 3 is, similar to the invention described in claim 1 or 2, the air-fuel ratio control means for performing the air-fuel ratio control based on the output of the oxygen sensor upstream of the three-way catalyst. And an air-fuel ratio control correction means for correcting the air-fuel ratio control based on the output of an oxygen sensor on the downstream side of the three-way catalyst, wherein the correction amount by the air-fuel ratio control correction means is A correction amount changing unit for changing and a deterioration degree detecting unit for detecting a deterioration degree of the three-way catalyst are provided, and the correction amount changing unit has a large deterioration degree of the three-way catalyst detected by the deterioration degree detecting unit. The smaller the correction amount, the smaller the correction amount is when the output of the downstream oxygen sensor is biased to the rich side of the stoichiometric air-fuel ratio compared to when it is biased to the lean side. Specially To.

According to the present invention, a combined operation of the operation of the invention described in claim 1 and the operation of the invention described in claim 2 can be obtained. Moreover, by acting simultaneously instead of acting individually, special mutual effects can be obtained. That is, when the correction from rich to lean is performed in a state where the degree of deterioration of the three-way catalyst is large, the correction amount is minimized, and the air-fuel ratio control is corrected with a slight suppression. In contrast, when the lean-to-rich correction is performed in a state where the degree of deterioration of the three-way catalyst is small, the correction amount is maximized and the air-fuel ratio control is corrected most favorably. Other than this, when performing correction from lean to rich in a state where the degree of deterioration of the three-way catalyst is large and when performing correction from rich to lean in a state where the degree of deterioration of the three-way catalyst is small,
The correction amount is set to the moderate level, and the air-fuel ratio control is corrected to the moderate level.

In any case, rational correction of the air-fuel ratio control is made according to the situation, and as a result, the catalyst does not become in an oxygen-depleted state or an oxygen-saturated state, and the optimum amount is sufficient. Oxygen is occluded and optimum purification by the three-way catalyst is achieved.

In the above description, "decreasing the correction amount" includes the case where the correction amount is zero, that is, the case where no correction is made. Hereinafter, the present invention will be described in more detail through embodiments of the invention.

[0024]

1 shows the system configuration of an engine 1 according to an embodiment of the present invention. A spark plug 3 is provided at the top of the combustion chamber defined by the piston 2. A fuel injection valve 5 is provided in an intake system 4 leading to the combustion chamber, and a three-way catalyst 7 and an upstream oxygen sensor 8 and a downstream oxygen sensor 9 are provided in an exhaust system 6 with the catalyst 7 in front and behind. ing. Both oxygen sensors 8 and 9 are so-called λO2 sensors, and the output voltage becomes high when the air-fuel ratio is richer than the stoichiometric air-fuel ratio (for example, about 1 V), and when the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. Is low (for example, about 0 V), and the output voltage greatly changes in the vicinity of the stoichiometric air-fuel ratio.

Engine control unit (ECU)
An air-fuel ratio signal (residual oxygen concentration signal) from both of the oxygen sensors 8 and 9 is input to 10 and a control signal is output to the fuel injection valve 5, the ignition plug 3, and the like. Although not shown, the ECU 10 has other basic parameters for engine control, such as an intake air amount signal from an air flow meter, a throttle opening signal from a throttle opening sensor, an engine rotation signal from an engine rotation sensor, and an engine water temperature. Various signals such as a cooling water temperature signal from the sensor are input.

[Fuel Injection Control] The air-fuel ratio of the engine 1 is controlled by the fuel injection amount from the fuel injection valve 5. The fuel injection amount is set as follows. First, the operating state of the engine 1 (intake air amount, engine rotation, cooling water temperature, oxygen sensor output, etc.) is detected, and the basic injection amount is calculated from the intake air amount and engine rotation. Then, a correction amount corresponding to the cooling water temperature and a feedback correction amount Cfb for converging the air-fuel ratio to the target air-fuel ratio are added to the basic injection amount.
(Described later) is added to calculate the final fuel injection amount.
Further, if necessary, an increase correction for warming up, a learning correction, or the like may be performed. Then, a drive pulse signal having a pulse width (considering the invalid pulse width) corresponding to the obtained fuel injection amount is output to the fuel injection valve 5 when the injection timing arrives.

[Air-fuel ratio feedback control] In this engine 1, the three-way catalyst 7 is operated when a predetermined execution permission condition is satisfied.
The feedback control of the air-fuel ratio based on the output E1 of the upstream oxygen sensor 8 is performed so as to maximize the exhaust gas purification performance. That is, the feedback correction amount Cf is set so that the deviation between the actual air-fuel ratio and the target air-fuel ratio is eliminated.
b is set. In that case, the air-fuel ratio is three way catalyst 7
The target air-fuel ratio is close to the stoichiometric air-fuel ratio (A / F = 14.7) so that the target air-fuel ratio falls within the window.

The feedback correction amount Cfb is set as follows. For example, immediately after the upstream side oxygen sensor 8 output E1 reverses from rich to lean, the rich proportional control gain P (plus value) is added to the feedback correction amount Cfb, and as a result, the fuel injection amount is on the rich side. To make an immediate increase. Thereafter, while the output E1 of the upstream oxygen sensor 8 is lean, the rich integral control gain I (plus value) is added to the feedback correction amount Cfb, and as a result, the fuel injection amount gradually increases to the rich side. To do so.

As a result, the upstream oxygen sensor 8
Immediately after the output E1 reverses from lean to rich,
The lean proportional control gain P (negative value) is added to the duck correction amount Cfb so that the fuel injection amount is immediately and immediately reduced to the lean side. After that, while the upstream side oxygen sensor 8 output E1 indicates rich, the lean integral control gain I (negative value) is added to the feedback correction amount Cfb, and as a result, the fuel injection amount gradually decreases to the lean side. To do so.

As a result, the upstream oxygen sensor 8
Immediately after the output E1 is inverted from rich to lean, the operation of adding the rich proportional control gain P to the feedback correction amount Cfb again is repeated.

As described above, since the air-fuel ratio of the exhaust gas flowing into the three-way catalyst 7 repeats rich-lean across the stoichiometric air-fuel ratio, the oxygen storage capacity (oxygen storage effect) Q of the three-way catalyst 7 becomes effective. By being utilized, the oxygen adsorption / desorption reaction is promoted, and the exhaust gas purification performance of the three-way catalyst 7 is maximized.

In the above, the case where the air-fuel ratio is feedback-controlled to the target air-fuel ratio by increasing / decreasing the fuel injection amount has been described, but instead of this, or together with this,
It is also possible to perform feedback control of the air-fuel ratio to the target air-fuel ratio by increasing / decreasing the intake air amount. The intake air amount can be increased or decreased by operating the throttle opening.

The above is the output E of the upstream oxygen sensor 8
The case where the enrichment or leaning of the air-fuel ratio is started immediately after 1 reverses from rich to lean or lean to rich has been described, but instead, the upstream side oxygen sensor 8 output E1 changes from rich to lean or lean. It is also possible to start enrichment or leaning of the air-fuel ratio after a lapse of a predetermined time T after reversing to rich (delay control). In this case, for example, if the delay time T becomes longer at the time of reversing from rich to lean, the air-fuel ratio becomes leaner accordingly, and conversely, if the delay time T becomes longer at the time of reversing from lean to rich, then that much. The air-fuel ratio will become rich.

[Air-fuel ratio feedback correction control] Catalyst 7
As it is used, it gradually deteriorates due to heat loss, sulfur poisoning, or phosphorus poisoning, the oxygen storage capacity Q decreases, and the purification optimum air-fuel ratio (window) fluctuates. As a result, when the catalyst 7 is depleted of oxygen and the air-fuel ratio deviates from the window to the rich side, the purification performance of HC and CO decreases, and conversely, the catalyst 7 is saturated with oxygen and deviates to the lean side. If so, the purification performance of NOx is reduced.

Therefore, in this engine 1, in order to compensate the deterioration of the exhaust gas purification performance due to the deterioration of the catalyst 7, the downstream side oxygen sensor 9 is used to detect the air-fuel ratio of the exhaust gas after passing through the catalyst 7. The air-fuel ratio feedback control is corrected based on the detection result. For example, when the output E2 of the downstream side sensor 9 is biased to the lean side of the stoichiometric air-fuel ratio, oxygen exceeds the oxygen storage capacity Q of the catalyst 7 and leaks out. The air-fuel ratio feedback control is corrected to the rich side because of the oxygen saturation state. As a result, the consumption of oxygen is promoted, the lean atmosphere is canceled out, and the oxygen storage capacity Q of the three-way catalyst 7 is effectively utilized again.

On the contrary, when the output E2 of the downstream side sensor 9 is deviated to the rich side of the stoichiometric air-fuel ratio, oxygen does not leak from the catalyst 7, so the catalyst 7 is in an oxygen depleted state. Therefore, the air-fuel ratio feedback control is corrected to the lean side. As a result, the storage of oxygen is promoted, the rich atmosphere is canceled, and the oxygen storage capacity Q of the three-way catalyst 7 is effectively utilized again.

Such correction of the air-fuel ratio feedback control is achieved by correcting the feedback correction amount Cfb. More specifically, the proportional control gain P that determines the skip amount of the feedback correction amount Cfb,
Similarly, the integral control gain I that determines the slope of the feedback correction amount Cfb can be increased or decreased to achieve the forced enrichment or forced lean of the air-fuel ratio. Further, the delay time T described above may be increased or decreased.

Alternatively, the air-fuel ratio feedback control can be corrected by correcting the output E1 of the upstream oxygen sensor 8 instead of or together with the feedback correction amount Cfb. In that case, if the output E1 of the sensor 8 is corrected to the lean side (the output voltage is corrected to a low value), the air-fuel ratio is forcibly enriched, and conversely, the output E1 of the sensor 8 is made rich. If the correction is made to the side (correction of the output voltage to a high value), the air-fuel ratio is forcibly made lean as a result.

FIG. 2 is a flowchart of the entire air-fuel ratio control operation of the engine 1. After inputting various signals from the above-mentioned sensors in step S1, step S2
Then, the degree of deterioration of the three-way catalyst 7 is detected. The method of detecting the degree of deterioration will be described later.

Next, at step S3, the correction amount Z for the air-fuel ratio feedback control is set. As described above, the correction amount Z is set to a value that corrects the air-fuel ratio feedback control to the lean side when the downstream oxygen sensor output E2 is biased to the rich side, and conversely, the downstream oxygen is set. When the sensor output E2 is biased to the lean side, the air-fuel ratio feedback control is set to a value that corrects to the rich side (basic characteristic A).

FIG. 3 shows the basic characteristic A of such a correction amount Z.
Is represented. When the downstream oxygen sensor output E2 is biased to the rich side, the correction amount Z is set to a negative value, and as a result, the fuel injection amount is reduced to the lean side. Conversely, when the downstream oxygen sensor output E2 is biased to the lean side, the correction amount Z is set to a positive value, and as a result, the fuel injection amount is increased to the rich side.

Returning to step S3 of FIG. 2, however, the correction amount Z becomes larger as (1) the deviation of the downstream oxygen sensor output E2 from the stoichiometric air-fuel ratio becomes larger (more specifically, the absolute value of the correction amount Z is (Large value). This characteristic is also represented by the basic characteristic A in FIG. That is, the larger the downstream oxygen sensor output E2 is biased toward the rich side, the smaller the correction amount Z is, and conversely, the more the downstream oxygen sensor output E2 is biased toward the lean side,
The correction amount Z is set to a larger positive value. As a result, even if the downstream oxygen sensor output E2 is largely deviated from the stoichiometric air-fuel ratio, the air-fuel ratio feedback control is promptly corrected, and as a result, the deviation of the air-fuel ratio from the window is promptly corrected, The deterioration of the purification performance of the catalyst 7 is promptly suppressed.

In this embodiment, as shown in FIG. 3, within a predetermined range in which the deviation of the downstream oxygen sensor output E2 from the stoichiometric air-fuel ratio is close to zero, the inclination of change in the correction amount Z is small. It has been moderate. As a result, overcorrection (overshoot) when the downstream oxygen sensor output E2 does not deviate significantly from the stoichiometric air-fuel ratio is prevented, and, for example, during correction from lean to rich, almost no oxygen is stored, It is suppressed that the catalyst 7 is in an oxygen-depleted state, or conversely, oxygen is excessively occluded during the correction from rich to lean and the catalyst 7 is in an oxygen saturated state.

Returning to step S3 in FIG. 2, however, the correction amount Z is set to a smaller value (more specifically, a smaller absolute value of the correction amount Z) as the degree of deterioration of the catalyst (2) becomes larger. This characteristic is represented in FIG. That is, the larger the degree of deterioration of the catalyst 7 is and the smaller the oxygen storage capacity Q is, the smaller the correction coefficient α for the correction amount Z (multiplied by the basic value of the correction amount Z shown in FIG. 3) is.

As a result, when the degree of deterioration of the catalyst 7 is large (when the oxygen storage capacity Q is small), the correction coefficient α has a small value, so that the lean-to-rich correction or the rich-to-lean correction is performed. As a result, the correction does not become excessive or excessive, and, for example, when correcting from lean to rich, most of the stored oxygen disappears, and the catalyst 7 becomes oxygen depleted, or conversely rich. When correcting from lean to lean, oxygen is excessively occluded,
It is suppressed that the catalyst 7 becomes oxygen saturated. Therefore, the catalyst 7 always stores an optimum amount of oxygen in a sufficient amount, and optimal purification by the three-way catalyst 7 is achieved.

On the contrary, when the degree of deterioration of the catalyst 7 is small and therefore the oxygen storage capacity Q is large, the correction coefficient α has a large value, so that either lean-to-rich correction or rich-to-lean correction is performed. It is considered to be accelerated, and as a result, there is no shortage of correction, for example, when correcting from lean to rich, the stored oxygen does not decrease so much,
The oxygen saturation state of the catalyst 7 is not resolved, or conversely, when correcting from rich to lean, oxygen is not stored so much,
It is suppressed that the oxygen-depleted state of the catalyst 7 is not eliminated. Therefore, also in this case, the catalyst 7 always stores the optimum amount of oxygen in a sufficient amount, and optimal purification by the three-way catalyst 7 is achieved.

Returning to step S3 in FIG. 2, however, the correction amount Z is (3) a large value during the lean-to-rich correction, as compared with the rich-to-lean correction (more specifically, the correction amount Z is an absolute value). The value is set to a large value (characteristic B).
This characteristic B is also represented in FIG. That is, the correction coefficient α used in the correction from lean to rich indicated by the solid line is always larger than the correction coefficient α used in the correction from rich to lean indicated by the broken line.

As a result, when the lean state is corrected to the rich side, in which oxygen is apparently leaking beyond the oxygen storage capacity Q of the catalyst 7, the rich correction is performed with an accelerating tendency.
The lean state is resolved within a short time, and the NOx emission amount is promptly suppressed.

On the other hand, although oxygen is still occluded,
Oxygen does not leak from the catalyst 7 as a result of being consumed for the oxidation of C and CO, or all of the stored oxygen is exhausted and the catalyst 7 becomes an oxygen depleted state. When it is corrected from the rich state to the lean side where it is not known whether or not it is not leaking from 7, the lean correction is performed with a slight suppression, so that the oxygen leaks from the catalyst 7 is prevented with the highest priority, and the NOx emission amount is reduced. The increase is suppressed.

In any case, as a result of the correction amount Z being set reasonably depending on the situation, the catalyst 7 is not in the oxygen-depleted state or the oxygen-saturated state, and the optimum amount is always sufficient. Oxygen is occluded, and optimum purification by the three-way catalyst 7 is achieved.

Returning to FIG. 2, finally, in step S4,
The air-fuel ratio feedback control described above is executed. Here, the feedback correction amount Cf is calculated by using the correction amount Z (for example, by adding it to the setting calculation formula of the fuel injection amount).
b, or the upstream oxygen sensor output E1 or the like is corrected. As a result, the feedback control of the air-fuel ratio is corrected by reflecting the correction amount Z.

The case where the basic characteristic A of the correction amount Z shown in FIG. 3 and the characteristic of the correction coefficient α with respect to the basic value of the correction amount Z shown in FIG. 4 are stored in the memory has been described above. Instead of this, as shown in FIG. 5, the basic characteristic of the correction amount Z obtained by combining both characteristics from the beginning may be stored in the memory.

From the above, in any case, the three-way catalyst 7
When the correction from rich to lean is performed in a state where the degree of deterioration of is large, the air-fuel ratio feedback control is corrected to the most restrained level. In contrast, when the lean-to-rich correction is performed in a state where the degree of deterioration of the three-way catalyst 7 is small, the air-fuel ratio feedback control is corrected most favorably. The air-fuel ratio feedback control is performed when the lean-to-rich correction is performed when the deterioration degree of the three-way catalyst 7 is large and when the correction is performed from the rich to lean when the deterioration degree of the three-way catalyst 7 is small. Is corrected to a moderate level. As a result, the air-fuel ratio feedback control is rationally corrected depending on the situation, and as a result, the catalyst 7 is not in an oxygen-depleted state or an oxygen-saturated state, and always stores an optimum amount of oxygen, which is sufficient. Optimal purification with a three-way catalyst is achieved.

In the above, when the correction amount Z is reduced, for example, the correction coefficient α may be set to zero and the correction amount Z may be set to zero so that the air-fuel ratio feedback control is not substantially corrected.

As the oxygen sensors 8 and 9, so-called linear O2 sensors whose output changes substantially linearly according to the air-fuel ratio may be adopted. In that case, for example, when the deviation of the downstream oxygen sensor output E2 from the stoichiometric air-fuel ratio becomes a predetermined value or more, the air-fuel ratio feedback control is corrected (a correction amount Z having a non-zero value is set). To
On the other hand, as described above, the oxygen sensors 8 and 9 have the λO2
In the case of a sensor, as shown in FIG. 3, if the downstream oxygen sensor output E2 deviates from the stoichiometric air-fuel ratio, the air-fuel ratio feedback control is corrected regardless of the magnitude of the deviation ( The correction amount Z having a value other than zero is set).

[Detection of Degree of Deterioration of Catalyst] Next, a method of detecting the degree of deterioration of catalyst, which is carried out in step S2, will be described. As described above, the degree of deterioration of the three-way catalyst can be generally detected by the oxygen storage capacity Q as a substitute characteristic. That is, as the catalyst deteriorates, a metal compound directly involved in the adsorption and desorption of oxygen (for example, cerium oxide CeO2) causes sintering (sintering), which reduces the contact area with oxygen and reduces oxygen storage. This is because the ability Q decreases.

FIG. 6 is an example of output waveforms of the upstream oxygen sensor 8 and the downstream oxygen sensor 9 when the air-fuel ratio is changed from rich to lean. As the air-fuel ratio is changed from rich to lean, the upstream sensor output E1 is inverted from lean to rich at time t1. On the other hand, the downstream sensor output E2 is the catalyst 7
Due to the oxygen storage capacity Q of, the lean is reversed to rich at time t2 which is later in time. This time interval Δt (= t
2-t1) or the difference in the output waveform (the area of the shaded region) represents the oxygen storage capacity Q of the catalyst 7.

For example, when the catalyst 7 is normal, it is assumed that the downstream sensor output E2 is inverted from lean to rich at time t2 as described above. When the oxygen storage capacity Q decreases with the deterioration of the catalyst 7, the reversal time of the downstream sensor output E2 gradually becomes earlier as shown by the arrow. Finally, as indicated by the symbol A, the upstream sensor output E1 and the downstream sensor output E2 are almost approximated to each other, the difference between the output waveforms disappears, and the oxygen storage capacity Q of the catalyst 7 disappears. Therefore, the degree of deterioration of the catalyst 7 at the present time can be detected depending on how much the time interval Δt or the difference in the output waveform of the catalyst 7 in the normal time is. It will be possible.

Next, FIG. 7 shows the output waveform of FIG. 6 more microscopically. The upstream sensor output E1 is frequently and periodically inverted between the rich side and the lean side by the air-fuel ratio feedback control. However, the downstream sensor output E2 is initially on the rich side because oxygen does not leak from the catalyst 7 due to the oxygen storage capacity Q of the catalyst 7. However, as the air-fuel ratio shifts to the lean side with the passage of time, oxygen exceeds the storage capacity of the catalyst 7 and leaks out, and the downstream side sensor output E2 shifts to the lean side, for example, as indicated by the symbol C. Causes the reversal of.
The difference between the output waveforms of both oxygen sensors at this time (area of shaded region: F) represents the oxygen storage capacity Q of the catalyst 7 at the present time, that is, the degree of catalyst deterioration. The same applies to the case indicated by the symbol K.

On the other hand, as indicated by the symbols S and S, the degree of deterioration of the catalyst 7 is also detected by the delay in the inversion of the downstream sensor output E2 to the rich side with respect to the inversion of the upstream sensor output E1 to the rich side. can do. in this case,
Oxygen is consumed for the oxidation of HC and CO generated by the rich, so that the downstream sensor output E2 is gradually inverted to the rich side. Then, the faster the speed of consumption of oxygen, in other words, the faster the speed of reversal of the downstream side sensor output E2 to the rich side, in other words, the difference between the two oxygen sensor output waveforms (the shaded area). Area of:
The oxygen storage capacity Q is lower and the degree of deterioration of the catalyst 7 is larger as the temperature is smaller.

In the symbol, the air-fuel ratio becomes leaner and oxygen always leaks from the catalyst 7. In this state, the upstream sensor output E1 and the downstream sensor output E2 substantially coincide with each other. This indicates that the change has occurred (state after time t2 in FIG. 6).

The detection of such a catalyst deterioration degree can be performed earlier in time than the conventional catalyst deterioration determination. That is, the degree of deterioration detection can be executed at any time when the air-fuel ratio is being inverted between rich and lean by feedback control of the air-fuel ratio, whereas in the conventional catalyst deterioration determination, as described above, for example, It cannot be determined that the catalyst has deteriorated until the value of the ratio of the number of reversals of the upstream sensor output E1 and the number of reversals of the downstream sensor output E2 becomes equal to or less than a predetermined value. Therefore, it takes a long time to determine that the catalyst has deteriorated, and no countermeasure is taken for the trouble caused by the deterioration of the catalyst which has been progressing until then. On the other hand, if the method of detecting the degree of deterioration of the catalyst as described above is adopted, measures are taken according to the degree of deterioration of the catalyst that is already in progress, without waiting for the determination of whether the catalyst has deteriorated or has not deteriorated. It can be taken at an early stage, and as a result, overall and comprehensive improvement of exhaust purification performance can be expected.

When the degree of catalyst deterioration exceeds a predetermined value, the malfunction indicator lamp (MIL) may be turned on to visually indicate to the user that the catalyst 7 is deteriorated.

Further, the downstream oxygen sensor 9 may be arranged at an intermediate position inside the catalyst 7. When the sensor output E2 reverses lean, it can be determined that about half of the oxygen has been stored in the catalyst 7. Alternatively, the catalyst 7 may be an underfloor catalyst, and a direct coupling catalyst may be separately installed on the upstream side of the exhaust system 6. Then, in that case, an upstream oxygen sensor may be installed upstream of the direct-coupled catalyst, and a downstream oxygen sensor may be installed between the direct-coupled catalyst and the underfloor catalyst to detect the degree of deterioration of the direct-coupled catalyst. . Further, in that case, the upstream-side directly coupled catalyst is a three-way catalyst, and the downstream-side underfloor catalyst is a so-called lean NOx that is excellent in NOx purification efficiency in a lean atmosphere, for example.
A catalyst may be adopted.

[0065]

As described above, according to the present invention, the air-fuel ratio control is performed based on the output of the upstream oxygen sensor, and the air-fuel ratio control is corrected based on the output of the downstream oxygen sensor. In the engine control system, the correction amount for the air-fuel ratio control is not uniform, but is changed according to the degree of deterioration of the three-way catalyst or the situation, and as a result, the reliability of the correction of the air-fuel ratio control is improved. The exhaust gas purification performance by the catalyst is further improved.

In particular, the above-mentioned method of detecting the degree of deterioration of the catalyst employed in the present invention uses the output of the downstream oxygen sensor in the so-called double oxygen sensor system, so that any vehicle model compatible with OBD can be used. It can be widely used in engines. The present invention can be expected to be widely applied to general engines having a plurality of oxygen sensors on the upstream side and the downstream side with a three-way catalyst interposed therebetween.

[Brief description of drawings]

FIG. 1 is a system configuration diagram of an engine according to an embodiment of the present invention.

FIG. 2 is a flowchart of an air-fuel ratio control operation of the engine.

FIG. 3 is a basic characteristic diagram of a correction amount Z for air-fuel ratio feedback control during the control operation.

FIG. 4 is a characteristic diagram of a correction coefficient α with respect to the same correction amount Z.

5 is a basic characteristic diagram of the correction amount Z obtained by combining the characteristics of FIG. 3 and the characteristics of FIG.

FIG. 6 is one of the catalyst deterioration degree detecting operations during the above control operation.
It is a time chart explaining an example.

FIG. 7 is a time chart illustrating another example.

[Explanation of symbols]

1 engine 5 Fuel injection valve 7 3-way catalyst 8 Upstream oxygen sensor 9 Downstream oxygen sensor 10 Engine control unit (ECU)

   ─────────────────────────────────────────────────── ─── Continued front page    F term (reference) 3G084 BA09 BA24 DA10 DA27 EB11                       FA07 FA10 FA20 FA30 FA33                 3G091 AA17 AA23 AA28 AB03 BA14                       BA15 BA19 BA33 CB02 DA01                       DA02 DB10 DC01 DC02 DC03                       EA01 EA05 EA16 EA34 FB10                       FB11 FB12 FC02 HA36 HA37                       HA42                 3G301 JA21 JB09 MA01 ND01 PA01Z                       PA11Z PD09A PD09Z PE01Z                       PE08Z

Claims (3)

[Claims] 1. An air-fuel ratio control means for controlling an air-fuel ratio of an air-fuel mixture supplied to a combustion chamber to a stoichiometric air-fuel ratio based on an output of an oxygen sensor upstream of a three-way catalyst arranged in an exhaust system of an engine. An air-fuel ratio control device for an engine, comprising: an air-fuel ratio control correction means for correcting the air-fuel ratio control by the control means based on an output of an oxygen sensor downstream of the three-way catalyst,
A correction amount changing means for changing the correction amount by the air-fuel ratio control correcting means and a deterioration degree detecting means for detecting the deterioration degree of the three-way catalyst are provided, and the correction amount changing means is detected by the deterioration degree detecting means. An air-fuel ratio control device for an engine, wherein the correction amount is reduced as the degree of deterioration of the generated three-way catalyst increases. 2. Air-fuel ratio control means for controlling the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber to the stoichiometric air-fuel ratio based on the output of an oxygen sensor upstream of the three-way catalyst arranged in the exhaust system of the engine. An air-fuel ratio control device for an engine, comprising: an air-fuel ratio control correction means for correcting the air-fuel ratio control by the control means based on an output of an oxygen sensor downstream of the three-way catalyst,
A correction amount changing unit for changing the correction amount by the air-fuel ratio control correcting unit is provided, and the changing unit is set to the lean side when the output of the downstream side oxygen sensor is biased to the rich side of the theoretical air-fuel ratio. An air-fuel ratio control device for an engine, wherein the correction amount is configured to be smaller than that in the case of deviation. 3. Air-fuel ratio control means for controlling the air-fuel ratio of the air-fuel mixture supplied to the combustion chamber to the stoichiometric air-fuel ratio based on the output of an oxygen sensor upstream of the three-way catalyst arranged in the exhaust system of the engine. An air-fuel ratio control device for an engine, comprising: an air-fuel ratio control correction means for correcting the air-fuel ratio control by the control means based on an output of an oxygen sensor downstream of the three-way catalyst,
A correction amount changing means for changing the correction amount by the air-fuel ratio control correcting means and a deterioration degree detecting means for detecting the deterioration degree of the three-way catalyst are provided, and the correction amount changing means is detected by the deterioration degree detecting means. The correction amount is reduced as the degree of deterioration of the determined three-way catalyst is increased, and when the output of the downstream side oxygen sensor is biased to the rich side of the stoichiometric air-fuel ratio,
An air-fuel ratio control device for an engine, wherein the correction amount is configured to be smaller than that in the case of being biased to the lean side.