JP2009074427A - Air-fuel ratio control device of internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To enhance convergence to a theoretical air-fuel ratio in air-fuel ratio feedback control using an oxygen sensor. <P>SOLUTION: A rich/lean against the theoretical air-fuel ratio is detected based on an output voltage V from the oxygen sensor. An air-fuel ratio feedback correction factor LAMBDA is so varied by a proportional integral operation that the air fuel ratio approaches the theoretical air-fuel ratio. The amount of integral I used for the proportional integral operation is varied larger as the output voltage V from the oxygen sensor is far separated from a theoretical air-fuel ratio equivalent value VS. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an air-fuel ratio control apparatus for an internal combustion engine, and more particularly to an apparatus for feedback-controlling the air-fuel ratio of an internal combustion engine using an oxygen sensor whose output changes suddenly at the theoretical air-fuel ratio.

Patent Document 1 discloses an air-fuel ratio control apparatus that burns an air-fuel mixture at a stoichiometric air-fuel ratio by proportional-integral control based on a rich / lean detection result by an oxygen sensor.
JP 2006-037875 A

By the way, in the oxygen sensor, only rich lean with respect to the stoichiometric air-fuel ratio is detected, and the deviation between the actual air-fuel ratio and the stoichiometric air-fuel ratio cannot be detected. The air-fuel ratio control signal is gradually changed by (Gain).
Here, if the integral is excessive, an air-fuel ratio overshoot occurs. Conversely, if the integral is excessively small, convergence to the theoretical air-fuel ratio is delayed, resulting in overshoot. It was difficult to converge to the theoretical air fuel ratio (target air fuel ratio) with good response while suppressing the above.

  The present invention has been made in view of the above problems, and an object of the present invention is to improve convergence to a theoretical air-fuel ratio in air-fuel ratio feedback control using an oxygen sensor.

Therefore, according to the first aspect of the present invention, in the air-fuel ratio feedback control based on rich lean by the oxygen sensor, the feedback gain is changed more greatly as the output of the oxygen sensor deviates from the stoichiometric air-fuel ratio equivalent value.
According to the above invention, when the output of the oxygen sensor is far from the stoichiometric air-fuel ratio equivalent value when compared with the stoichiometric air-fuel ratio equivalent value, the air-fuel ratio control signal is changed with a larger feedback gain, Accelerates convergence to the theoretical air-fuel ratio.

According to a second aspect of the present invention, as the temperature of the internal combustion engine is lower, an increase in the feedback gain due to the output of the oxygen sensor is suppressed.
According to the above invention, even if the output of the oxygen sensor is the same, if the engine temperature at that time is low, a smaller feedback gain is set. , Avoid the occurrence of overshoot.

According to the third aspect of the present invention, the output range of the oxygen sensor is learned, and the feedback gain is changed according to the learned output range.
According to the above invention, when the minimum output value or the maximum output value of the oxygen sensor changes due to deterioration and the output range changes, the feedback gain is changed based on the changed output range.
In the invention according to claim 4, the feedback gain corresponding to the output of the oxygen sensor is changed to be smaller with respect to the deterioration (delay) of the response characteristic of the oxygen sensor.

According to the above invention, when the response of the oxygen sensor is deteriorated, the feedback gain set in accordance with the output of the oxygen sensor is lowered as a whole to avoid the occurrence of overshoot.
According to the fifth aspect of the present invention, when the rich error / lean error during transient operation of the internal combustion engine is resolved, the air-fuel ratio is reversed, and the air-fuel ratio control signal returns toward the reference value, the feedback gain is stored in advance. The fixed value was set.

  According to the above invention, due to the transient operation of the internal combustion engine, a lean error (a shift of the air-fuel ratio toward the lean side) occurs during acceleration, and a rich error (a shift of the air-fuel ratio toward the rich side) occurs during deceleration, eliminating this air-fuel ratio error Therefore, when the air-fuel ratio is inverted as a result of changing the air-fuel ratio control signal, the feedback gain is set to a fixed value stored in advance instead of a value corresponding to the output of the oxygen sensor. The air-fuel ratio control signal is changed toward the reference value.

According to the sixth aspect of the invention, the rich error / lean error state during the transient operation of the internal combustion engine indicates that the air-fuel ratio inversion period, the integrated value of the transient correction amount of the fuel injection amount, the air-fuel ratio control signal and the reference value Judgment was made based on at least one of the deviations.
According to the above invention, the rich error / lean during transient operation is based on at least one of the air-fuel ratio inversion period, the integrated value of the transient correction amount of the fuel injection amount, and the deviation between the air-fuel ratio control signal and the reference value. If it is a rich error / lean error state during transient operation, it is determined whether or not it is in an error state. When the air-fuel ratio control signal returns toward the reference value, the feedback gain is set to a fixed value stored in advance. .

According to a seventh aspect of the present invention, the feedback gain is stored in advance until the rich error / lean error during transient operation of the internal combustion engine is resolved, the air-fuel ratio is reversed, and the air-fuel ratio control signal returns to the reference value. The fixed value was made.
According to the above invention, a rich error / lean error occurs due to the transient operation of the internal combustion engine, and when the air-fuel ratio is inverted as a result of changing the air-fuel ratio control signal to eliminate the air-fuel ratio error, the feedback gain is changed to the oxygen sensor. The feedback gain, which is a fixed value, is used until the air-fuel ratio control signal returns to the reference value.

In the invention according to claim 8, when the air-fuel ratio is reversed again before the air-fuel ratio control signal returns to the reference value, the feedback gain is returned to a value corresponding to the output of the oxygen sensor.
According to the above-described invention, when the air-fuel ratio is reversed while returning the air-fuel ratio control signal to the reference value with the fixed feedback gain, the feedback control is performed with an excessive gain to cause hunting or overshoot. In order to prevent this, the feedback gain is returned from a fixed value to a value corresponding to the output of the oxygen sensor.

In the invention according to claim 9, as the response of the oxygen sensor further deteriorates, the upper limit value of the feedback gain is set smaller, and the feedback gain set according to the output of the oxygen sensor is limited to the upper limit value or less. I tried to do it.
According to the above invention, when the response of the oxygen sensor is deteriorated and the inversion of the output of the oxygen sensor is delayed with respect to the actual inversion of the air-fuel ratio, an overshoot (overcorrection state) is likely to occur. As the progress proceeds, the upper limit value of the feedback gain is set to a smaller value, and the feedback gain set according to the output of the oxygen sensor is limited so as not to exceed the upper limit value.

Embodiments of the present invention will be described below.
FIG. 1 shows a system configuration of an air-fuel ratio control apparatus for an internal combustion engine in the embodiment.
In FIG. 1, an intake pipe 12 of an internal combustion engine 11 is provided with an air flow meter 13 for detecting an intake air flow rate QA and a throttle valve 14 for controlling the intake air flow rate in conjunction with an accelerator pedal.

A fuel injection valve 15 is provided for each cylinder in the intake manifold downstream of the throttle valve 14.
The fuel injection valve 15 is driven to open by an injection pulse signal output from the engine control unit 50, and injects fuel controlled to a predetermined pressure.
Further, a water temperature sensor 16 for detecting the coolant temperature TW in the cooling jacket of the internal combustion engine 11, a crank angle sensor 20 for detecting the angle of the crankshaft, a throttle sensor 21 for detecting the opening of the throttle valve 14, and the like are provided. .

The engine control unit 50 calculates the engine speed NE based on the signal output from the crank angle sensor 20.
On the other hand, the exhaust pipe 17 is provided with a three-way catalytic converter 19 that purifies exhaust by oxidizing CO and HC and reducing NOx.
Further, the exhaust pipe 17 upstream of the three-way catalytic converter 19 is provided with an oxygen sensor 18 whose output changes abruptly at the stoichiometric air-fuel ratio.

  The oxygen sensor 18 is formed, for example, by coating the inner and outer surfaces of a zirconia tube with electrodes and platinum that acts as a catalyst. Between the inner side (atmosphere side) and the outer side (exhaust side) of the zirconia tube, the oxygen sensor 18 FIG. 2 shows an oxygen concentration cell that generates an electromotive force according to the ratio to the oxygen concentration. As shown in FIG. 2, the electromotive force is higher on the rich side than the stoichiometric air-fuel ratio, and on the lean side than the stoichiometric air-fuel ratio. Is a characteristic of lowering.

However, the structure of the oxygen sensor 18 is not limited to the one provided with the zirconia tube as long as the output changes suddenly at the stoichiometric air-fuel ratio. For example, a plate-type sensor may be used. .
The engine control unit 50 includes a microcomputer comprising a CPU, a ROM, a RAM, an A / D converter, an input / output interface, and the like. The oxygen sensor 18, the air flow meter 13, the water temperature sensor 16, the crank described above. Detection signals from the angle sensor 20 and the throttle sensor 21 are input to control the fuel injection amount TI by the fuel injection valve 15.

  The fuel injection amount TI includes a basic fuel injection amount TP, a transient correction amount KAT for increasing / decreasing the fuel injection amount during acceleration / deceleration, and an effective valve opening time fluctuating due to an operation delay with respect to the injection pulse of the fuel injection valve 15. Based on the voltage correction amount TS to be corrected and the air-fuel ratio feedback correction coefficient LAMBDA (air-fuel ratio control signal) for bringing the actual air-fuel ratio closer to the theoretical air-fuel ratio, it is calculated as TI = (TP + KAT) × LAMBDA + TS.

The basic fuel injection pulse width TP is calculated from the intake air flow rate QA detected by the air flow meter 13, the engine rotational speed NE obtained from the signal of the crank angle sensor 20, the coolant temperature TW, and the like.
The air / fuel ratio feedback correction coefficient LAMBDA is calculated based on the rich / lean value with respect to the stoichiometric air / fuel ratio detected by the oxygen sensor 18.

  In the calculation of the air-fuel ratio feedback correction coefficient LAMBDA, first, the output voltage V of the oxygen sensor 18 is compared with a threshold voltage VS corresponding to the theoretical air-fuel ratio, and the output voltage V of the oxygen sensor 18 is compared with the threshold voltage VS. If the output voltage V of the oxygen sensor 18 is lower than the threshold voltage VS, the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. (See FIG. 2).

  Then, as shown in FIG. 3, when the air-fuel ratio is inverted from lean to rich (when the output voltage V increases and changes across the threshold voltage VS), the air-fuel ratio feedback correction coefficient LAMBDA is decreased stepwise by a proportional amount P. After that, until the air-fuel ratio reverses lean (until the output voltage V decreases and changes across the threshold voltage VS), the air-fuel ratio feedback correction coefficient LAMBDA is gradually decreased and changed with the slope of the integral I. .

  When the fuel injection amount TI is decreased by the decrease change of the air-fuel ratio feedback correction coefficient LAMBDA and the air-fuel ratio is reversed to lean (when the output voltage V changes to decrease across the threshold voltage VS), this time, the air-fuel ratio feedback correction coefficient LAMBDA is increased in a stepwise manner by a proportional amount P, and thereafter, it is emptied with a slope of the integral I until the air-fuel ratio reverses richly (until the output voltage V changes across the threshold voltage VS). The fuel ratio feedback correction coefficient LAMBDA is gradually increased (see FIG. 3).

The calculation function of the air-fuel ratio feedback correction coefficient LAMBDA by the proportional integration operation by the engine control unit 50 corresponds to the feedback control means in this embodiment.
Note that the reference value (initial value) of the air-fuel ratio feedback correction coefficient LAMBDA is set in advance to 1.0 that does not substantially perform increase / decrease correction.

Here, the setting process (software function corresponding to gain changing means) of the proportional component P and integral component I corresponding to the feedback gain by the engine control unit 50 will be described with reference to the flowcharts of FIGS. The routines shown in the flowcharts of FIGS. 4 and 6 are executed every predetermined minute time.
In the flowchart of FIG. 4, in step S101, it is determined whether an execution condition for air-fuel ratio feedback is satisfied.

As the air-fuel ratio feedback condition, it is determined that the engine load / engine speed range is in a predetermined range, the oxygen sensor 18 is activated, the fuel is not being cut, and the like.
The predetermined engine load is an engine load region excluding a load region where combustion other than the stoichiometric air-fuel ratio is required, such as a load region where an output air-fuel ratio is required, and the predetermined engine rotation speed region is This is a rotation region excluding a high rotation region where fuel increase is performed to suppress an increase in exhaust temperature.

  In principle, the air-fuel ratio feedback execution condition is a predetermined engine load / engine speed range, the oxygen sensor 18 is activated, and fuel is not being cut. In addition, it is not determined that the execution condition of the air-fuel ratio feedback is satisfied when all of the plurality of conditions are satisfied, but the execution condition of the air-fuel ratio feedback is not satisfied when a part of the plurality of conditions is satisfied. The establishment can be determined.

If the air-fuel ratio feedback condition is satisfied, the process proceeds to step S102, and the coolant temperature TW detected by the water temperature sensor 16 and the output voltage V of the oxygen sensor 18 are read.
In the next step S103, a response diagnosis of the oxygen sensor 18 is performed.
In the response diagnosis, for example, the output voltage V of the oxygen sensor 18 or the cycle of the air-fuel ratio feedback correction coefficient LAMBDA when the air-fuel ratio feedback control state is in a steady state is measured, and the cycle and the operating condition ( It is possible to determine that the response of the oxygen sensor 18 is deteriorated as the actual cycle is longer than the reference cycle.

The oxygen sensor 18 can also be detected by detecting the changing speed (slope) of the output voltage V of the oxygen sensor 18 and comparing the detected speed with a reference speed according to operating conditions (engine load / engine speed). Can be diagnosed.
Further, the air-fuel ratio is stepped so as to straddle the theoretical air-fuel ratio, and the time until this air-fuel ratio step change is detected by the oxygen sensor 18 is measured, and the measurement time and the operating conditions (engine load / engine The response of the oxygen sensor 18 can also be diagnosed by comparing with a reference time corresponding to the rotation speed), and all known methods for response diagnosis can be applied.

When the response diagnosis of the oxygen sensor 18 is performed in step S103, in step S104, the proportional integral of the air-fuel ratio feedback correction coefficient LAMBDA is based on the cooling water temperature TW (engine temperature) and the output voltage V of the oxygen sensor 18 at that time. A proportional coefficient P and an integral coefficient I correction coefficient KIP used for control are set.
Specifically, in the central partial region including the threshold voltage VS in the output range of the output voltage V, the correction coefficient KIP is a constant value, and deviates from the central partial region and approaches the minimum value or the maximum value. The correction coefficient KIP is gradually set larger.

As will be described later, the result of multiplying the correction coefficient KIP by the basic value of the proportional component P and integral component I is set as the final proportional component P and integral component I, and the correction factor KIP is large. The proportional component P and the integral component I, which are feedback gains, are set larger.
In other words, a larger proportional component P and integral component I (feedback gain) are set as the output voltage V of the oxygen sensor 18 departs from the threshold voltage VS corresponding to the theoretical air-fuel ratio.

  When the output voltage V of the oxygen sensor 18 is far from the threshold voltage VS, the deviation between the actual air-fuel ratio and the stoichiometric air-fuel ratio is unknown, but not at least immediately before the rich-lean of the air-fuel ratio is reversed, At this time, if the air-fuel ratio feedback correction coefficient LAMBDA is increased or decreased by a large integral I, the convergence to the theoretical air-fuel ratio is accelerated. On the other hand, if the output voltage V approaches the threshold voltage VS, the integral I is more increased. Since the change is made small, the occurrence of overshoot can be suppressed (see FIG. 7).

The correction coefficient KIP is generally set lower as the coolant temperature TW (engine temperature) is lower.
When the cooling water temperature TW (intake port temperature) is low, the response delay of the air-fuel ratio control increases due to an increase in the amount of fuel flowing through the wall, etc. Therefore, the same proportional amount P, integral as when the cooling water temperature TW is high At the minute I (feedback gain), overshoot is likely to occur.

Therefore, the correction coefficient KIP is set to be larger as the output voltage V of the oxygen sensor 18 is farther from the threshold voltage VS, and the correction coefficient KIP is made smaller as the cooling water temperature TW is lower.
The proportional component P is a value used when the output voltage V of the oxygen sensor 18 is close to the threshold voltage VS, and thus is not substantially corrected in accordance with the output voltage V of the oxygen sensor 18. The increase / decrease is corrected according to the water temperature TW.

In addition, the water temperature representative of the engine temperature is used for setting the correction coefficient KIP (proportional component P, integral component I). The correctness of the correction coefficient KIP (proportional component P, integral component I) is determined by the intake port portion. Since it depends on the temperature, the temperature of the intake port can be detected instead of the water temperature.
By the way, in step S104, as described above, in order to determine the correction coefficient KIP based on the output voltage V of the oxygen sensor 18, a table in which the output voltage V and the correction coefficient KIP are made to correspond one-to-one in advance is stored. (See FIG. 8).

  However, when the oxygen sensor 18 deteriorates with time, the minimum voltage value floats and / or the maximum voltage value decreases. When the correction coefficient KIP is determined by referring to the table as it is in such a deterioration state, for example, the same minimum voltage Even if the value is stuck, a different correction coefficient KIP is set between the initial state and the state where the minimum voltage value is floated due to deterioration. Set to a small value.

  For example, when the output changes in the initial state of the oxygen sensor 18 within the range of 0V to 1V, but the output range changes from 50mV to 1V due to the float of the minimum output value due to deterioration, the output in the initial state If the correlation between the range and the feedback gain is applied even after deterioration, the feedback gain corresponding to the output range of 0 V to 50 mV will not be adopted as a result, and even if the feedback gain is stuck to the minimum output value, It will be kept lower than the initial state.

  For this reason, in the initial state, if the state is stuck to the minimum voltage value, the air-fuel ratio feedback correction coefficient LAMBDA is increased or decreased by a relatively large integral I (feedback gain). When the minimum voltage value rises, the integral I (feedback gain) to be set becomes smaller, and as a result, the convergence to the theoretical air-fuel ratio (target air-fuel ratio) decreases.

Therefore, the output range of the oxygen sensor 18 is learned so that an appropriate feedback gain is assigned in the output range after the deterioration of the oxygen sensor 18, and when the output range changes, the output range changes according to the changed output range. The feedback gain was changed.
Specifically, when the execution condition of the air-fuel ratio feedback is not satisfied, in step S110, the minimum / maximum value (output range) of the output voltage V of the oxygen sensor 18 is learned, and the correction is performed in the actual output range. In order to use up the variable range of the coefficient KIP, the output voltage V is converted according to the actual minimum and maximum values (output range) at that time when referring to the table.

The flowchart of FIG. 5 shows in detail the learning of the minimum and maximum values (output range) of the output voltage V in step S110.
In step S201, it is determined whether or not the fuel is being cut to stop fuel injection by the fuel injection valve 15.
In the present embodiment, for example, fuel cut (stop of fuel injection) is performed during deceleration operation, and in such a fuel cut state, air flows directly into the exhaust pipe, and the exhaust air-fuel ratio becomes extremely lean. Thus, the output voltage V of the oxygen sensor 18 indicates the minimum value of the output range.

Therefore, if it is determined in step S201 that the fuel is being cut, the process proceeds to step S202, and it is determined whether or not the current value is smaller than the previous value of the output voltage V of the oxygen sensor 18.
If the current value is smaller than the previous value, the process proceeds to step S203, where the current value is set to the minimum value VMIN of the output voltage V. If the current value is larger than the previous value, the process bypasses step S203. As a result, the minimum value VMIN is not updated.

That is, when the output voltage V decreases with the start of the fuel cut, the minimum value VMIN is successively updated and stored, and the output voltage V at the time when it finally decreases is learned as the minimum value VMIN.
On the other hand, if it is determined in step S201 that the fuel cut is not in progress, the routine proceeds to step S204, where it is determined whether or not a full increase is being performed.

The full increase is a state in which the fuel injection amount is significantly increased from the stoichiometric air-fuel ratio when the accelerator is fully open or close to it or when an output air-fuel mixture is requested, and the increase level is predetermined. Indicates that the value is greater than or equal to the value.
In addition, although the air-fuel ratio feedback control is being performed, for example, a case in which a large increase correction is performed with acceleration can be included in the full increase.

In the full increase state, the air-fuel ratio becomes significantly richer than the stoichiometric air-fuel ratio, so that the output voltage V of the oxygen sensor 18 shows the maximum value of the output range.
Therefore, if it is determined in step S204 that the full increase is being performed, the process proceeds to step S205, where it is determined whether or not the current value is larger than the previous value of the output voltage V of the oxygen sensor 18.
If the current value is larger than the previous value, the process proceeds to step S206, where the current value is set to the maximum value VMAX of the output voltage V. If the current value is smaller than the previous value, the process bypasses step S206. As a result, the maximum value VMAX is not updated.

That is, when the output voltage V increases with the start of full increase, the maximum value VMAX is successively updated and stored, and the output voltage V at the time when it finally becomes the highest is learned as the maximum value VMAX. .
When the minimum value VMIN is obtained as described above, the output range sandwiched between the minimum value VMIN and the minimum side reference value (100 mV) is the minimum value (for example, 0 V) and the minimum side reference value (100 mV) in the initial state. For example, the learned minimum value VMIN is always converted to 0V, and the correction coefficient KIP corresponding to 0V is obtained.

  Similarly, when the maximum value VMAX is obtained, the output range sandwiched between the maximum value VMAX and the maximum side reference value (700 mV) is the initial value (for example, 850 V) and the maximum side reference value (700 mV). For example, the learned maximum value VMAX is always converted to 850 V, and a correction coefficient KIP corresponding to 850 V is obtained (see FIG. 8).

As described above, when the correction factor KIP for the proportional component P and the integral component I is set according to the output voltage V of the oxygen sensor 18 and the coolant temperature TW (engine temperature), in the next step S105, the oxygen sensor 18 is set. Is set based on the result of the response diagnosis performed in step S103.
The correction coefficient KR is set to a smaller value as the response deterioration of the oxygen sensor 18 progresses (the response is slower).

When the response of the oxygen sensor 18 is deteriorated, the output change of the oxygen sensor 18 with respect to the change of the exhaust air-fuel ratio (exhaust oxygen concentration) is delayed. As a result, overshoot is likely to occur. The integral I (feedback gain) is corrected so as to avoid the occurrence of overshoot.
Instead of the correction by the correction coefficient KR, the upper limit values of the proportional component P and the integral component I are set to smaller values as the response deterioration progresses, and the proportional component P and the integral component I are limited to the upper limit value or less. Further, the upper limit value can be reduced as the cooling water temperature TW (engine temperature) at that time is lower (see FIG. 9).

In step S106, the final proportional component P and integral component I are obtained by multiplying the basic values of the proportional component P and integral component I by the correction coefficients KIP and KR (final P, I = basic P, I × KIP × KR).
Here, the basic values of the proportional component P and the integral component I may be fixed values, or may be variably set according to the engine load and the engine speed.

When the basic values of the proportional component P and the integral component I are fixed values, the basic values of the proportional component P and the integral component I are calculated from the output voltage V of the oxygen sensor 18 and the water temperature TW, and the basic values are obtained from the sensor. Correction can be made with a correction coefficient KR corresponding to the response, or the basic value can be limited by an upper limit value corresponding to the sensor response.
When the upper limit value is set according to the degree of deterioration of the oxygen sensor 18, the result obtained by multiplying the basic value of the proportional component P and the integral component I by the correction coefficient KIP is limited to the upper limit value or less. The subsequent values are the final proportional component P and integral component I.

It should be noted that in the setting of the proportional component P and the integral component I, the proportional component P and the integral component I may be changed according to at least the output voltage V of the oxygen sensor 18, and the proportional component P and integral depending on the water temperature TW are sufficient. It is possible to omit one or all of the correction process for the minute I, the conversion process for dealing with the change in the maximum / minimum value due to the deterioration, and the correction process for the response deterioration.
In step S107, 1 is set to a flag FKATO that is set to “1” in a predetermined period immediately after the air-fuel ratio error state due to the transient operation is resolved and the air-fuel ratio rich / lean is reversed. Judge whether or not.

If FKATO = 0, the process bypasses step S108 and proceeds to step S109, where the air-fuel ratio feedback correction coefficient LAMBDA is calculated using the proportional component P and integral component I set in step S106.
On the other hand, when FKATO = 1, the process proceeds to step S108, where the proportional value P and integral value I, which are fixed values stored in advance, are used as final values instead of the calculation result in step S106, and the process proceeds to step S109.

  When a large rich error / lean error occurs due to the acceleration / deceleration operation of the internal combustion engine, the air / fuel ratio feedback correction coefficient LAMBDA is changed to the reference value (1.0 by changing the air / fuel ratio feedback correction coefficient LAMBDA until the error state is resolved. ), But the actual air-fuel ratio has already been reversed before the oxygen sensor 18 detects the reversal of the air-fuel ratio, so the reversal of the air-fuel ratio is detected by the oxygen sensor 18. After that, while the air-fuel ratio feedback correction coefficient LAMBDA returns to the reference value, an overcorrection state occurs.

Therefore, after the air-fuel ratio is reversed, in order to quickly return the air-fuel ratio feedback correction coefficient LAMBDA, it is not a proportional component P or an integral component I (feedback gain) determined according to the output of the oxygen sensor 18, but a larger value. A fixed value is used to quickly eliminate the overcorrection state (see FIG. 10).
The flowchart of FIG. 6 shows the setting process of the flag FKATO.

In step S301, as in step S101, it is determined whether an execution condition for air-fuel ratio feedback is satisfied.
Here, if the execution condition of the air-fuel ratio feedback is satisfied, the process proceeds to step S302, and it is determined whether or not the elapsed time t from the previous rich / lean inversion exceeds the reference time ts. The reference time ts is variably set according to the engine load / engine speed.

If the elapsed time t from the previous rich / lean inversion exceeds the reference time ts, the process proceeds to step S303, where the integrated value of the transient correction KAT from the previous rich / lean inversion becomes the predetermined value α. Determine whether it has exceeded.
If the integrated value of the transient correction KAT from the previous rich / lean inversion exceeds the predetermined value, the process further proceeds to step S304, where the air-fuel ratio feedback correction coefficient LAMBDA and the reference value (= 1.0) It is determined whether or not the absolute value of the deviation exceeds a predetermined value β.

When all of the above three conditions of Step S302 to Step S304 are satisfied, a large air-fuel ratio error occurs due to the transient operation of the internal combustion engine 11, and a large correction amount is required for a long time to return this to the stoichiometric air-fuel ratio. Therefore, it is judged that a large change in the air-fuel ratio feedback correction coefficient LAMBDA was required.
When a large air-fuel ratio error or a fuel correction value shift occurs due to the transient operation of the internal combustion engine 11, the output of the oxygen sensor 18 sticks to the maximum or minimum, and the air-fuel ratio feedback correction coefficient LAMBDA is rapidly changed with a large feedback gain. When the output of the oxygen sensor 18 is reversed as a result of correcting the fuel injection amount to increase / decrease and at the same time correcting the fuel injection amount in the direction to eliminate the air-fuel ratio error also by the transient correction KAT, an overcorrection state is entered. As a result, the convergence to the stoichiometric air-fuel ratio is reduced.

Therefore, in order to quickly eliminate the excessive correction state, the proportional component P and integral component I, which are fixed values larger than the proportional component P and integral component I set in step S106, are set in step S108. .
When it is determined that all the three conditions of Steps S302 to S304 are satisfied, the process proceeds to Step S305, and 1 is set to a flag FK indicating that the air-fuel ratio error state is in transient operation.

Since it is only necessary to determine the air-fuel ratio error state associated with the transient operation of the internal combustion engine 11, it is not necessary to use all three conditions of S302 to S304. For example, one or two of the three conditions are used. Can be determined.
Further, the transient operation state of the engine 11 can be added to the condition. For example, when the transient operation is performed and the air-fuel ratio inversion interval time is long, or when the transient operation is performed and the air-fuel ratio is When the deviation between the feedback correction coefficient LAMBDA and the reference value is large, 1 can be set in the flag FK.

In step S306, it is determined whether the rich / lean air-fuel ratio detected by the oxygen sensor 18 has been reversed.
When the reversal of the air-fuel ratio is detected by the oxygen sensor 18, the process proceeds to step S307, where it is determined whether 1 is set in the flag FKATO.
If the rich / lean air-fuel ratio is reversed with the flag FKATO = 1, the process proceeds to step S308 to reset the flag FKATO to zero.

On the other hand, if it is determined in step S307 that the flag FKATO = 0, the process proceeds to step S309, where it is determined whether 1 is set in the flag FK.
If the flag FK is set to 1, it is determined that the air-fuel ratio error state associated with the transient operation has been eliminated and the air-fuel ratio has been reversed, and the process proceeds to step S310, where 1 is set to the flag FKATO. .

On the other hand, when the flag FK = 0, the current reversal of the air-fuel ratio is not the result of correcting the air-fuel ratio error due to the transient operation, but the result of the periodic correction process of the air-fuel ratio in the steady operation. Determination is made to bypass step S310 and proceed to step S311.
From step S308 or step S310, the process proceeds to step S311, and the elapsed time t from the air-fuel ratio inversion, the integrated value of the transient correction KAT, and the flag FK are reset to 0, respectively.

On the other hand, if it is determined in step S306 that the air-fuel ratio is not rich / lean, the process proceeds to step S312, and it is determined whether 1 is set in the flag FKATO.
If the flag FKATO = 1, the process proceeds to step S313, and it is determined whether or not the air-fuel ratio feedback correction coefficient LAMBDA has reached the reference value 1.0 (crossed the reference value 1.0). .

If it is determined that the air-fuel ratio feedback correction coefficient LAMBDA has reached the reference value 1.0 (crosses the reference value 1.0), the process proceeds to step S314, and the flag FKATO is reset to 0. .
That is, when the air-fuel ratio error state during transient operation is resolved and the air-fuel ratio is reversed, 1 is set in the flag FKATO, and then the air-fuel ratio is reversed again or the air-fuel ratio feedback correction coefficient LAMBDA is the reference value. When the value returns to 1.0 (crosses the reference value 1.0), the flag FKATO is reset to 0 (see FIG. 10).

  In the state where 1 is set in the flag FKATO, the proportional component P and the integral component I (feedback gain) are set to fixed values larger than the values determined in accordance with the output of the oxygen sensor 18, so that the transient operation is performed. The excessive correction state after the air-fuel ratio error state at the time is canceled and the air-fuel ratio is reversed can be quickly eliminated by the proportional component P and the integral component I (feedback gain) as the fixed values.

  Further, after the flag FKATO is set to 1, the flag FKATO is reset to 0 when the air-fuel ratio is reversed again or the air-fuel ratio feedback correction coefficient LAMBDA reaches the reference value, and the proportional component P and integral component I ( (Feedback gain) is returned to the normal value corresponding to the output of the oxygen sensor 18, it is possible to avoid excessive feedback from continuing with excessive gain, and to prevent occurrence of hunting or the like.

  The proportional component P and integral component I (feedback gain) used as fixed values when the flag FKATO = 1 is set to a smaller value as time passes or as the air-fuel ratio feedback correction coefficient LAMBDA approaches the reference value. In addition, the flag FKATO can be reset to 0 when the air-fuel ratio feedback correction coefficient LAMBDA returns to within a predetermined range including the reference value after the flag FKATO = 1.

Here, technical ideas other than the claims that can be grasped from the above embodiment will be described together with the effects thereof.
(A) The feedback control means sets an air-fuel ratio control signal including an integration operation for increasing / decreasing the air-fuel ratio control signal in increments of integral based on rich lean with respect to the theoretical air-fuel ratio,
The air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 1 to 9, wherein the gain changing unit sets the integral according to an output of the oxygen sensor.

According to the above invention, if the air-fuel ratio is rich, the air-fuel ratio control signal is decreased by an integral amount to make the air-fuel ratio lean, and if the air-fuel ratio is lean, the air-fuel ratio control signal is increased by an integral amount. The air-fuel ratio is enriched so that it approaches the stoichiometric air-fuel ratio, but the integral is set according to the output of the oxygen sensor.
(B) The gain changing means learns the output range of the oxygen sensor from the output of the oxygen sensor in a fuel cut state and / or the output of the oxygen sensor in a full fuel increase state. The air-fuel ratio control apparatus for an internal combustion engine according to claim 3.

  According to the above-described invention, in the fuel cut state, the air-fuel ratio becomes the super lean state, the output of the oxygen sensor indicates the lean side limit value, and in the fuel full increase state, the air fuel ratio becomes the theoretical sky. It becomes richer than the fuel ratio, and the output of the oxygen sensor shows the limit value on the rich side. From the limit value on the lean side and / or the limit value on the rich side, the output range of the oxygen sensor at that time To learn.

1 is a system configuration diagram of an internal combustion engine in an embodiment. The diagram which shows the correlation with the air fuel ratio in embodiment, and the output of an oxygen sensor. 6 is a timing chart showing basic characteristics of proportional-integral control based on rich / lean determination with respect to the theoretical air-fuel ratio in the embodiment. The flowchart which shows the mode of the air-fuel ratio feedback control including the setting process of the proportional component P and the integral component I in embodiment. The flowchart which shows the learning process of the output range of the oxygen sensor in embodiment. The flowchart which shows the setting process of the flag FKATO which shows the overcorrection state after the air fuel ratio error by transient operation in embodiment. 6 is a timing chart showing changes in the air-fuel ratio feedback correction coefficient LAMBDA when the gain is changed according to the output of the oxygen sensor in the embodiment. The diagram which shows the setting characteristic of the gain according to the output when the output range of an oxygen sensor changes in embodiment. The diagram which shows the response of the oxygen sensor in embodiment, and the characteristic of the gain upper limit with respect to water temperature. The timing chart which shows the integration operation in the overcorrection state after the air fuel ratio error by transient operation in the embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 11 ... Internal combustion engine, 13 ... Air flow meter, 15 ... Fuel injection valve, 18 ... Oxygen sensor, 19 ... Three-way catalyst, 20 ... Crank angle sensor, 50 ... Control unit

Claims (9)

An oxygen sensor arranged in the exhaust pipe of the internal combustion engine, whose output changes suddenly at the theoretical air-fuel ratio,
Feedback control means for detecting rich / lean relative to the stoichiometric air-fuel ratio based on the output of the oxygen sensor and outputting an air-fuel ratio control signal so that the air-fuel ratio approaches the stoichiometric air-fuel ratio;
Gain changing means for changing the feedback gain in the feedback control means to a greater extent as the output of the oxygen sensor deviates from the stoichiometric air-fuel ratio equivalent value;
An air-fuel ratio control apparatus for an internal combustion engine, comprising:   2. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the gain changing means suppresses an increase change in the feedback gain due to the output of the oxygen sensor as the temperature of the internal combustion engine is lower.   The air-fuel ratio control apparatus for an internal combustion engine according to claim 1 or 2, wherein the gain changing means learns an output range of the oxygen sensor and changes the feedback gain according to the learned output range.   The gain changing means changes the feedback gain according to the output of the oxygen sensor to be smaller with respect to the deterioration of the response of the oxygen sensor. An air-fuel ratio control apparatus for an internal combustion engine.   The gain changing means stores the feedback gain in advance when the rich error / lean error during transient operation of the internal combustion engine is resolved, the air-fuel ratio is reversed, and the air-fuel ratio control signal returns toward the reference value. The air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 1 to 4, wherein the air-fuel ratio control apparatus has a fixed value.   When the gain changing means is in a rich error / lean error state during transient operation of the internal combustion engine, an air-fuel ratio inversion period, an integrated value of the transient correction amount of the fuel injection amount, an air-fuel ratio control signal, and a reference value 6. The air-fuel ratio control apparatus for an internal combustion engine according to claim 5, wherein the determination is made based on at least one of the deviations.   The gain changing means stores the feedback gain in advance until the rich error / lean error during transient operation of the internal combustion engine is resolved, the air-fuel ratio is reversed, and the air-fuel ratio control signal returns to the reference value. 6. The air-fuel ratio control apparatus for an internal combustion engine according to claim 5, wherein the air-fuel ratio control apparatus has a fixed value.   The gain changing means returns the feedback gain to a value corresponding to the output of the oxygen sensor when the air-fuel ratio is inverted again before the air-fuel ratio control signal returns to a reference value. The air-fuel ratio control apparatus for an internal combustion engine according to claim 7.   The gain changing means sets the upper limit value of the feedback gain smaller as the deterioration of the response of the oxygen sensor progresses, and limits the feedback gain set according to the output of the oxygen sensor to the upper limit value or less. The air-fuel ratio control apparatus for an internal combustion engine according to claim 4.

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