JP2009074397A - Air-fuel ratio control device for internal combustion engine - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To accurately carry out leanness for early activation of a catalyst in an engine provided with an oxygen sensor capable of detecting only rich/lean to stoichiometric air-fuel ratio. <P>SOLUTION: Air-fuel ratio feedback control changing an air-fuel ratio feedback correction coefficient LAMBDA by proportional integration action is carried out at first after engine start based on a rich/lean detection result by the oxygen sensor, and correction demand for keeping air fuel ratio at the stoichiometric air fuel ratio is determined. A clamp value for making air-fuel ratio lean by multiplying the correction demand for keeping the stoichiometric air-fuel ratio by a correction coefficient K, and the air-fuel ratio feedback correction coefficient LAMBDA is clamped to the clamp value. The air-fuel ratio is kept leaner then stoichiometric air-fuel ratio and activation of the catalyst is accelerated during clamp. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to an air-fuel ratio control device for an internal combustion engine, and more particularly, to a technique for activating a catalyst for purifying exhaust gas at an early stage by making the air-fuel ratio leaner than the stoichiometric air-fuel ratio.

  In Patent Document 1, when the engine is started, a catalyst activity determination water temperature is set based on the cooling water temperature at that time, and after the start until a predetermined time elapses, an increase after start is performed. A control device for an internal combustion engine is disclosed that performs control to return the air-fuel ratio to the stoichiometric air-fuel ratio when the water temperature reaches the catalyst activity determination water temperature in the predetermined lean state by gradually leaning the fuel ratio. Yes.

When the air-fuel ratio is made leaner than the stoichiometric air-fuel ratio, the oxygen concentration in the catalyst atmosphere becomes high and the oxidation reaction in the catalyst is promoted. As a result, the catalyst temperature rises and early activation is achieved.
JP 09-151759 A

By the way, in order to activate the catalyst at an early stage, when the air-fuel ratio is made lean, if the air-fuel ratio is made excessively lean, the combustion stability is impaired, and as a result, the engine speed greatly fluctuates. Further, if the air-fuel ratio is not sufficiently lean, the time until the catalyst is activated becomes long, so that the accuracy of leaning the air-fuel ratio is required.
In the control device of Patent Document 1, the fuel loss correction amount for leaning is determined based on the water temperature at the start, and in such a configuration, depending on the properties of the fuel used at that time and the engine state There is a possibility that the lean air-fuel ratio varies greatly, and the above-described fluctuation in rotation and a decrease in the catalyst activation effect occur.

  On the other hand, in an engine equipped with an air-fuel ratio sensor that detects the air-fuel ratio in a wide range, it is possible to accurately control the target lean air-fuel ratio by feedback control based on the detection result of the air-fuel ratio sensor. In an engine equipped with an oxygen sensor that can detect only rich and lean, feedback control with the stoichiometric air-fuel ratio as the target air-fuel ratio can be performed, but the oxygen sensor does not show an output change according to the air-fuel ratio change in the lean region, Since it is not possible to detect how much lean air-fuel ratio is, it is not possible to detect a deviation between the target lean air-fuel ratio and the actual air-fuel ratio, and feedback control based on the deviation cannot be performed. The fuel ratio must be made lean.

However, with feedforward control, it is difficult to accurately control the target lean air-fuel ratio.Excessive leaning causes combustion to become unstable, resulting in fluctuations in engine speed, However, if it is controlled to the rich side, the effect of the catalytic activity cannot be sufficiently obtained.
The present invention has been made in view of the above problems, and in an engine equipped with an oxygen sensor that can detect only the rich lean relative to the stoichiometric air-fuel ratio, the leaning for early activation of the catalyst can be performed with high accuracy. The purpose is to do.

  Therefore, the invention described in claim 1 includes air-fuel ratio feedback control means for calculating and outputting an air-fuel ratio control signal so that the actual air-fuel ratio approaches the stoichiometric air-fuel ratio based on the rich / lean detection result by the oxygen sensor. In the internal combustion engine, the air-fuel ratio is made lean based on the air-fuel ratio control signal in the first predetermined period after the calculation and output of the air-fuel ratio control signal by the air-fuel ratio feedback control means is started after the internal combustion engine is started. A clamp value of the air-fuel ratio control signal to be held is determined, and the air-fuel ratio control signal is clamped to the clamp value in a second predetermined period following the first predetermined period.

According to the above invention, as a result of calculating / outputting the air / fuel ratio control signal based on the rich / lean detected by the oxygen sensor, when the actual air / fuel ratio fluctuates around the theoretical air / fuel ratio, the air / fuel ratio control signal The average value (center value) indicates a correction value required for obtaining the theoretical air-fuel ratio.
Therefore, the air-fuel ratio feedback based on the detection result of the oxygen sensor is performed only for the first predetermined period, so that the correction request for making the actual air-fuel ratio the stoichiometric air-fuel ratio in the current situation is known in advance, and the stoichiometric air-fuel ratio is obtained. By determining a clamp value for maintaining a lean air-fuel ratio with reference to a correction request to achieve, the air-fuel ratio in the second predetermined period that is the clamp period is controlled to be lean with respect to the theoretical air-fuel ratio. To do.

Therefore, even if the correction request for the same air-fuel ratio differs depending on the fuel properties and engine conditions, the leaning allowance is determined based on the correction request by knowing the correction request for the theoretical air-fuel ratio in advance. The lean control for promoting the catalyst activity can be performed with high accuracy, and the catalyst activity can be promoted to the maximum by generating the rotation fluctuation.
According to a second aspect of the present invention, in the first aspect of the present invention, when a rich state is detected by the oxygen sensor in the second predetermined period, the clamp value of the air-fuel ratio control signal is set to a direction in which the air-fuel ratio becomes leaner. It was corrected to.

According to the above invention, the oxygen sensor is characterized by being richer than the stoichiometric air-fuel ratio in the second predetermined period to be held leaner than the stoichiometric air-fuel ratio by clamping the air-fuel ratio control signal to the clamp value. Is detected, the clamp value is corrected in the direction in which the air-fuel ratio becomes lean, so that the lean state can be realized.
Therefore, even if the air-fuel ratio during clamping becomes rich due to disturbance (change in fuel wall flow rate due to changes in engine temperature, etc.), it can immediately return to the lean state and promote catalyst activity.

In the invention of claim 3, in the invention of claim 1 or 2, the air-fuel ratio control signal is clamped on condition that the internal combustion engine is in an idle state.
According to the above invention, when the accelerator is opened and the engine is shifted to the non-idle state while the air-fuel ratio control signal is clamped in the idle state, the operation at the lean air-fuel ratio is canceled by canceling the clamp. The

Therefore, it is possible to avoid operating at a lean air-fuel ratio when the engine is in a non-idle state and leaning for catalyst activity is unnecessary.
According to a fourth aspect of the invention, in the invention according to any one of the first to third aspects, the clamp value is determined based on an average value of the air-fuel ratio control signal in the last one cycle in the first predetermined period. I tried to do it.

According to the above invention, the average value in the last one cycle immediately before the air-fuel ratio control signal is clamped is obtained, and the clamp value is set from the average value.
At the beginning of air-fuel ratio feedback control, it will be affected by the amount of fuel increase at start-up and cold-down time, and the correction request from the air-fuel ratio control signal at that time to the theoretical air-fuel ratio cannot be judged with high accuracy. The clamp value is set from the value just before clamping the air-fuel ratio feedback control.

  Further, in the air-fuel ratio feedback control calculated based on the rich / lean determination, the actual air-fuel ratio is controlled so that the average air-fuel ratio becomes the stoichiometric air-fuel ratio by alternately repeating the rich state and the lean state. Therefore, since the average value of the air-fuel ratio control signal at this time can be regarded as a correction request for making the air-fuel ratio coincide with the theoretical air-fuel ratio, the average value in the last one cycle is calculated as the theoretical value. Obtained as a correction request for matching with the air-fuel ratio, and a clamp value for achieving a lean state is determined based on this correction request.

Accordingly, the clamp value for realizing lean operation for early activation of the catalyst can be set with high accuracy, and by clamping to the clamp value, the lean air-fuel ratio in the clamped state can be controlled with high accuracy. Thus, the catalytic activity can be effectively promoted.
According to a fifth aspect of the present invention, in the first aspect of the present invention, the end of the first predetermined period is determined based on the number of rich / lean reversals of the air-fuel ratio. .

According to the above invention, the air-fuel ratio repeats the rich state and the lean state alternately with the air-fuel ratio feedback control, and when the number of inversions of the rich lean reaches a predetermined number, the clamp value is determined at that time. Then, the second predetermined period for clamping the air-fuel ratio control signal is shifted.
By repeating the rich / lean of the air / fuel ratio a predetermined number of times, it is possible to accurately determine a correction request for making the stoichiometric air / fuel ratio from the air / fuel ratio control signal at that time, and a clamp value for controlling to a constant lean air / fuel ratio Can be set with high accuracy.

According to a sixth aspect of the invention, in the invention according to any one of the first to fifth aspects, the increase correction of the fuel injection amount is canceled during the first predetermined period and the second predetermined period.
According to the above invention, in the configuration in which the fuel injection amount is corrected to be increased after startup or in a cold state, when the air-fuel ratio feedback control is performed in the first predetermined period, the increase correction is canceled, so that the air-fuel ratio control signal Can be avoided from excessively changing in the leaning direction, and a correction request for achieving the stoichiometric air-fuel ratio at an early stage can be calculated.

Accordingly, the start of lean control (clamping) for activating the catalyst early by shortening the first predetermined period can be accelerated, so that the activity of the catalyst can be further accelerated.
Furthermore, if the increase correction is canceled in the second predetermined period during which clamping is performed, the air-fuel ratio during clamping can be stably controlled, and catalyst activity can be effectively promoted.
According to a seventh aspect of the invention, in the invention according to any one of the first to sixth aspects, the clamp is stopped when it is determined that the exhaust purification catalyst is activated.

According to the above invention, when it is determined that the exhaust purification catalyst is activated after the start of clamping, the clamping is stopped at that time and the air-fuel ratio feedback control is resumed.
Leaning by clamping the air-fuel ratio control signal promotes the activity of the exhaust purification catalyst, but leaning is unnecessary after the exhaust purification catalyst is activated, so the clamping is stopped and the stoichiometric air-fuel ratio is reached. Give feedback control.

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 internal combustion engine 11 is a spark ignition gasoline engine for a vehicle.
The intake pipe 12 of the internal combustion engine 11 is provided with an air flow meter 13 for detecting the 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 adjusted to a predetermined pressure into the intake port.
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 (exhaust purification catalyst) that purifies exhaust gas by oxidizing CO and HC and reducing NOx. The three-way catalytic converter 19 has, for example, a layer obtained by mixing platinum Pt and rhodium Rh on the surface of an alumina carrier.

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, and may be, for example, a plate-type sensor or the like, and may be provided with a heater.
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 is a basic fuel injection amount TP calculated so as to generate a stoichiometric air-fuel mixture with the cylinder intake air amount at that time, an increase amount at start and after start, and an increase amount at low water temperature. , Various correction factors COEF that are set including the increase amount at high load, the increase / decrease correction amount at acceleration / deceleration, the voltage correction amount TS for correcting the valve opening delay due to the battery voltage, and the actual air-fuel ratio Based on an air-fuel ratio feedback correction coefficient LAMBDA (air-fuel ratio control signal) for approaching the fuel ratio, it is calculated as TI = TP × COEF × LAMBDA + TS.

The air / fuel ratio feedback correction coefficient LAMBDA is calculated based on rich / lean relative 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 the threshold voltage S / L corresponding to the theoretical air-fuel ratio, and the output voltage V of the oxygen sensor 18 is the threshold voltage. If it is higher than S / L, it is determined that the air-fuel ratio is richer than the stoichiometric air-fuel ratio. If the output voltage V of the oxygen sensor 18 is lower than the threshold voltage S / L, the air-fuel ratio is It is determined that the air-fuel ratio is leaner than the theoretical air-fuel ratio (see FIG. 2).

  As shown in FIG. 3, when the air-fuel ratio reverses from lean to rich (when the output voltage V increases and changes across the threshold voltage S / L), the air-fuel ratio feedback correction coefficient LAMBDA is stepped by a proportional amount P. After that, until the air-fuel ratio reverses lean (until the output voltage V changes across the threshold voltage S / L), the air-fuel ratio feedback correction coefficient LAMBDA is gradually increased with the slope of the integral I. Change to decrease.

  When the air-fuel ratio is reversed from rich to lean by reducing the fuel injection amount TI by reducing the air-fuel ratio feedback correction coefficient LAMBDA (when the output voltage V changes across the threshold voltage S / L), this time, The fuel ratio feedback correction coefficient LAMBDA is increased and changed stepwise by a proportional amount P, and then integrated until the air-fuel ratio reverses rich (until the output voltage V increases and crosses the threshold voltage S / L). The air-fuel ratio feedback correction coefficient LAMBDA is gradually increased and changed with the slope of the minute I (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 air-fuel ratio feedback control means in this embodiment.
The proportional component P and the integral component I corresponding to the feedback gain may be fixed values or can be variably set according to the engine operating state such as the engine load and the engine speed.

In addition, 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 of the fuel injection amount.
By the way, the three-way catalytic converter 19 (exhaust gas purification catalyst) increases both the CO and HC conversion rate and the NOx conversion rate when the engine 11 is operated at the stoichiometric air-fuel ratio. If it is not activated when the temperature exceeds a predetermined temperature, the intended conversion performance is not exhibited, and CO, HC and NOx are not sufficiently purified but discharged into the atmosphere.

Therefore, at the time of cold start of the engine 11, it is desired to activate the three-way catalytic converter 19 at an early stage. In this embodiment, in order to activate the three-way catalytic converter 19 at an early stage, the air-fuel ratio is set to the theoretical sky. A control is performed to increase the oxygen concentration in the catalyst atmosphere by making the fuel leaner than the fuel ratio and to promote the oxidation reaction in the catalyst.
Hereinafter, lean control (function as a clamping means) for early catalyst activation by the engine control unit 50 will be described in detail with reference to the flowcharts of FIGS.

The routine shown in the flowchart of FIG. 4 is started when the ignition switch is turned on. First, in step S101, it is determined whether a start condition for air-fuel ratio feedback control is satisfied.
As start conditions for the air-fuel ratio feedback control, it is determined that the engine 11 is not being started and that the oxygen sensor 18 is activated.

Here, when the output of the oxygen sensor 18 generates a rich output of a predetermined value or more, or when the coolant temperature (engine temperature) is equal to or higher than the predetermined temperature, the active state of the oxygen sensor 18 can be estimated. it can.
In the present embodiment, the start condition of the air-fuel ratio feedback is determined not when the engine 11 is started and when the oxygen sensor 18 is activated. However, the present invention is not limited to this. In addition, it is not determined that the execution condition of the air-fuel ratio feedback is satisfied when all of the multiple conditions are satisfied, but is determined that the execution condition of the air-fuel ratio feedback is satisfied when a part of the multiple conditions is satisfied. Can do.

Until the start condition of the air-fuel ratio feedback control is satisfied, by repeating the determination in step S101, by various correction coefficients COEF set including an increase amount at the start and after the start, an increase amount at the low water temperature, etc. The fuel injection amount is corrected so as to increase more than the stoichiometric air-fuel ratio equivalent amount, and the engine is operated at a rich air-fuel ratio than the stoichiometric air-fuel ratio (see FIG. 6).
Until the air-fuel ratio feedback control is started, the air-fuel ratio feedback correction coefficient LAMBDA is held at the reference value of 1.0.

  On the other hand, when the oxygen sensor 18 is activated after the engine 11 is started and it is determined that the start condition of the air-fuel ratio feedback control is satisfied, the process proceeds to step S102, and rich / lean with respect to the stoichiometric air-fuel ratio is calculated based on the output of the oxygen sensor 18. An air-fuel ratio feedback control is performed in which the air-fuel ratio feedback correction coefficient LAMBDA is changed by a proportional integration operation based on the determination result.

When the air-fuel ratio feedback control is executed, the various correction coefficients COEF are lowered to values stored in advance to cancel the normal increase correction, and the values are held until the end of the clamp described later ( (See FIG. 6).
This is because when the air-fuel ratio is feedback-controlled to the stoichiometric air-fuel ratio in the increase correction state with various correction factors COEF, it takes time to converge to the vicinity of the stoichiometric air-fuel ratio, and the air-fuel ratio feedback control in the clamped state This is because the air-fuel ratio control accuracy decreases.

Moreover, it is preferable to use a dedicated threshold voltage as the threshold voltage S / L for rich / lean determination because the output characteristics of the oxygen sensor 18 are unstable immediately after the activation of the oxygen sensor 18.
After the start of the air-fuel ratio feedback control, it is determined in step S103 whether or not a condition (clamping condition) for clamping the air-fuel ratio feedback correction coefficient LAMBDA to enter the open control state is satisfied.

  Here, a predetermined time has elapsed since the start, the engine 11 is idling (accelerator fully closed), and the air-fuel ratio has been reversed a predetermined number of times after the start of the air-fuel ratio feedback control. When the fuel ratio feedback correction coefficient LAMBDA is repeatedly increased or decreased a predetermined number of times), it is determined that the clamp condition is satisfied.

The clamping condition is not limited to the three conditions, and the clamping can be executed when only at least one of the three conditions is satisfied.
During the period (first predetermined period) after the start of air-fuel ratio feedback until the clamp condition is satisfied, air-fuel ratio feedback control based on the output of the oxygen sensor 18 (calculation / output by proportional / integral operation of the air-fuel ratio feedback correction coefficient LAMBDA) If the clamping condition is satisfied, the process proceeds to step S104.

The first predetermined period is a period sufficient for the rich / lean output of the oxygen sensor 18 to be stably repeated centering around the theoretical air-fuel ratio, and is predetermined by actual machine verification, simulation, or the like.
In step S104, the clamp value for clamping the air-fuel ratio feedback correction coefficient LAMBDA is calculated.

Specifically, the average value of the air-fuel ratio feedback correction coefficient LAMBDA in the last one cycle immediately before the clamp condition is satisfied is multiplied by a previously stored correction coefficient K (<1.0), and the result is used as the clamp value. (Clamp value = average value of LAMBDA × correction coefficient K).
In the calculation of the average value, the value immediately before the air-fuel ratio feedback correction coefficient LAMBDA is corrected to be increased by the proportional operation is successively updated and stored as the minimum value, and the value immediately before the decrease correction by the proportional operation is set as the maximum value. The average value of the maximum and minimum values (average value = (maximum value + minimum value) / 2) when the condition is satisfied is obtained as the average value (center value) of the air-fuel ratio feedback correction coefficient LAMBDA in the last one cycle. .

The average value of the air-fuel ratio feedback correction coefficient LAMBDA indicates a correction request for making the actual air-fuel ratio coincide with the theoretical air-fuel ratio, and the air-fuel ratio feedback correction coefficient LAMBDA is actually clamped to the average value. It is possible to make the air / fuel ratio substantially equal to the theoretical air / fuel ratio.
Then, if the average value is multiplied by the correction coefficient K and the value corrected to decrease is used as the clamp value, and the air-fuel ratio feedback correction coefficient LAMBDA is clamped to the clamp value, the air-fuel ratio is made lean by a certain percentage from the theoretical air-fuel ratio. I will let you.

For example, when the clamp value of the air-fuel ratio feedback correction coefficient LAMBDA is set to a predetermined fixed value, the air-fuel ratio obtained with the fixed value varies depending on the difference in fuel properties.
However, as described above, if the correction request for matching the stoichiometric air-fuel ratio is known, and the clamp value is determined based on the correction request, the correction request for matching the stoichiometric air-fuel ratio is the difference in fuel properties, etc. Therefore, the clamp value is a value that can be controlled to a substantially constant lean air-fuel ratio without being affected by differences in fuel properties.

When the air-fuel ratio sensor capable of detecting the air-fuel ratio in a wide range is provided, when the catalyst is activated early with the air-fuel ratio in the lean state, feedback control can be performed with the lean air-fuel ratio as the target air-fuel ratio.
In contrast to this, in the engine 11 that performs air-fuel ratio feedback control using the oxygen sensor 18 as in this embodiment, feedback control with the target air-fuel ratio as the lean air-fuel ratio cannot be performed, but feedback control to the stoichiometric air-fuel ratio is possible. With the configuration in which the clamp value for setting the lean state is determined based on the air-fuel ratio feedback correction coefficient LAMBDA at the time, the lean air-fuel ratio can be controlled with necessary and sufficient accuracy.

When the clamp value is determined in step S104, the air-fuel ratio feedback correction coefficient LAMBDA is clamped to the clamp value in the next step S105, and the air-fuel ratio control is set to the open control state (feedback control stop state) (see FIG. 6). ).
That is, the air-fuel ratio feedback control is started only for a predetermined period after the start (first predetermined period), the clamp value is determined from the air-fuel ratio feedback correction coefficient LAMBDA at this time, and the predetermined period (the first period following the execution period of the air-fuel ratio feedback control) 2), the air-fuel ratio feedback correction coefficient LAMBDA is clamped to the clamp value, and the air-fuel ratio is controlled to be lean during the clamp period (second predetermined period).

In the clamp period (second predetermined period) in which the air-fuel ratio is controlled to be lean, the oxygen concentration in the atmosphere (inflow exhaust) of the three-way catalytic converter 19 increases, the oxidation reaction in the catalyst is promoted, and the catalyst temperature rises. As a result, the catalyst activity is accelerated compared to the operation at the stoichiometric air-fuel ratio.
If the three-way catalytic converter 19 is activated early, CO, HC and NOx can be purified at a high conversion rate from an early stage after the start, and the amount of CO, HC and NOx discharged at the start can be reduced.

The second predetermined period can be terminated if it is estimated that the three-way catalytic converter 19 has been activated, and the end period can be determined by determining the catalyst activity, or a predetermined value determined in advance from an actual machine or simulation. It can be done.
During the clamping, it is basically clamped to the clamp value calculated in step S104. However, in this embodiment, when the air-fuel ratio becomes rich due to disturbance during clamping, the clamp value is corrected to a lean state. The clamp value correction control will be described with reference to the flowchart of FIG.

Note that the routine shown in the flowchart of FIG. 5 shows the details of the processing in step S105 of the flowchart of FIG.
In the flowchart of FIG. 5, in step S201, it is determined whether the output voltage V of the oxygen sensor 18 exceeds the threshold voltage S / L and the rich determination is made in the clamped state of the air-fuel ratio feedback correction coefficient LAMBDA.

  When the rich determination is made, the process proceeds to step S202, where correction for decreasing the clamp value at a constant speed, that is, correction for changing the air-fuel ratio in the leaning direction is started. In the next step S203, the decrease of the clamp value is started. It is determined whether or not the output voltage V of the oxygen sensor 18 has fallen below the threshold voltage S / L due to the correction, in other words, whether or not it has returned to the lean state.

Until the air-fuel ratio returns to the lean state, the clamp value decrease correction is continued. When it is detected that the lean state has been returned, the process proceeds to step S204 to stop the clamp value decrease correction. The clamp value is held thereafter.
As described above, if the air / fuel ratio becomes richer than the stoichiometric air / fuel ratio due to some disturbance during clamping, if the clamp value is corrected to decrease and returned to the lean state, it will remain rich due to the disturbance. Thus, the catalyst activity can be reliably accelerated by lean operation.

In step S106, it is determined whether or not the condition for stopping the clamping of the air-fuel ratio feedback correction coefficient LAMBDA is satisfied.
Here, the fluctuation of the engine rotation speed has become a predetermined value or more, the engine rotation speed has decreased to a predetermined value or more, the idling operation state has been removed, and the cooling water temperature has exceeded the predetermined temperature (catalyst) If any one of the clamp stop conditions such as (estimated activation) is satisfied, the process proceeds to step S108 to start normal control (air-fuel ratio feedback control) and stop the clamp.

The condition for stopping the clamp based on the engine rotation speed is to ensure the stability of the engine operation by returning to the operation at the stoichiometric air-fuel ratio when the rotation becomes unstable due to the deterioration of the combustion stability due to the lean operation. is there.
In addition, when the engine 11 is operated at a medium or high load, the exhaust flow rate / exhaust temperature is increased and the catalyst activity is promoted and the lean operation is continued. This is because there is no need to make them.

Further, when it is estimated that the cooling water temperature is higher than the predetermined temperature and the catalyst is activated, the necessity for promoting the activity is eliminated, so that the clamping is stopped.
The catalyst activity can be determined based on the cooling water temperature or by directly detecting the catalyst temperature with a temperature sensor.
On the other hand, if the clamp stop condition is not satisfied, the process proceeds to step S107 to determine whether the clamp end condition is satisfied.

For example, when the elapsed time from the start of the clamp has reached a predetermined time, it is determined that the clamp end condition is satisfied, and the process proceeds to step S108 to start normal control (air-fuel ratio feedback control), thereby terminating the clamp. .
In other words, the clamp of the air-fuel ratio feedback correction coefficient LAMBDA is performed for a predetermined time, but if the stop condition is satisfied before the predetermined time is reached, the clamp is stopped before the predetermined time is reached. By clamping the air-fuel ratio feedback correction coefficient LAMBDA in the period (second predetermined period) after the start of clamping until the stop or end determination is made, the air-fuel ratio is maintained in a lean state, and the catalyst Early activation can be achieved.

  The control for early activation of the catalyst by the lean operation is particularly effective in an engine in which the catalyst activity is delayed with respect to the activation of the oxygen sensor 18. For example, in the V-type or horizontally opposed engine, the exhaust manifold of each bank is controlled. When the oxygen sensor is arranged at the gathering portion and the catalyst is arranged after the exhaust pipes of the banks merge, the catalyst activity is delayed with respect to the activity of the oxygen sensor.

Here, technical ideas other than the claims that can be grasped from the above embodiment will be described together with the effects thereof.
(B) The air-fuel ratio control apparatus for an internal combustion engine according to claim 4, wherein the clamp means determines the clamp value as a result of multiplying the average value by a correction coefficient (<1.0).
According to the above invention, the average value of the air-fuel ratio control signal indicates a correction request for matching the stoichiometric air-fuel ratio, and this average value is multiplied by a correction coefficient (<1.0) so that it is constant based on the stoichiometric air-fuel ratio. A clamp value for leaning the air-fuel ratio by a ratio is set.

Therefore, even if there is variation in the correction request for obtaining the stoichiometric air-fuel ratio, the air-fuel ratio during clamping can be controlled to a constant lean air-fuel ratio.
(B) The internal combustion engine according to any one of claims 1 to 7, wherein the clamping means stops the clamping when the engine rotational speed fluctuates more than a predetermined value. Air-fuel ratio control device.

According to the invention, when the air-fuel ratio control signal is clamped and controlled to be lean, if the engine rotational speed fluctuates more than a predetermined value, the combustion stability is reduced due to the lean air-fuel ratio. Therefore, the stability of combustion is recovered by stopping the clamp and returning to the operation at the stoichiometric air-fuel ratio.
Therefore, when the combustion stability is lowered by the lean clamp, it is not left as it is, and the stability of the engine operation can be ensured.

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 control for the early activation of the catalyst in embodiment. The flowchart which shows correction | amendment control of the clamp value in the clamp in embodiment. 4 is a time chart showing the state of an air-fuel ratio feedback correction coefficient LAMBDA in an air-fuel ratio feedback control period and a subsequent clamp period 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 catalytic converter, 50 ... Control unit

Claims (7)

An oxygen sensor disposed in an exhaust pipe of an internal combustion engine for detecting rich / lean relative to the stoichiometric air-fuel ratio;
An exhaust purification catalyst disposed in an exhaust pipe downstream of the oxygen sensor;
An air-fuel ratio feedback control means for calculating and outputting an air-fuel ratio control signal so that the actual air-fuel ratio approaches the stoichiometric air-fuel ratio based on rich lean detected by the oxygen sensor;
The air-fuel ratio for maintaining the air-fuel ratio lean based on the air-fuel ratio control signal in a first predetermined period after the calculation / output of the air-fuel ratio control signal by the air-fuel ratio feedback control means is started after the internal combustion engine is started. Clamping means for determining a clamp value of the air-fuel ratio control signal and clamping the air-fuel ratio control signal to the clamp value in a second predetermined period following the first predetermined period;
An air-fuel ratio control apparatus for an internal combustion engine, comprising:   2. The internal combustion engine according to claim 1, wherein, when the oxygen sensor detects a rich state in the second predetermined period, the clamp unit corrects the clamp value in a direction in which the air-fuel ratio becomes leaner. Engine air-fuel ratio control device.   3. The air-fuel ratio control apparatus for an internal combustion engine according to claim 1, wherein the clamping means clamps the air-fuel ratio control signal on condition that the internal combustion engine is in an idle state.   The said clamp means determines the said clamp value based on the average value of the air fuel ratio control signal in the last 1 period in the said 1st predetermined period, The one of Claims 1-3 characterized by the above-mentioned. An air-fuel ratio control apparatus for an internal combustion engine.   5. The internal combustion engine air condition according to claim 1, wherein the clamping means determines the end of the first predetermined period based on the number of times of rich-lean reversal of the air-fuel ratio. Fuel ratio control device.   6. The air-fuel ratio control for an internal combustion engine according to claim 1, wherein the clamping means cancels the increase correction of the fuel injection amount during the first predetermined period and the second predetermined period. apparatus.   The air-fuel ratio control apparatus for an internal combustion engine according to any one of claims 1 to 6, wherein the clamp means stops the clamp when it is determined that the exhaust purification catalyst is activated.

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