JP3230401B2 - Engine air-fuel ratio control device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air-fuel ratio control device for operating an engine lean (lean mixture).

[0002]

2. Description of the Related Art At the same time as improving fuel efficiency of an engine, NO
In order to reduce x, the fuel supply amount is controlled so that the air-fuel ratio, which is the ratio of air and fuel, becomes a lean air-fuel ratio leaner than the stoichiometric air-fuel ratio. An engine operating method for switching from lean operation to operation at the stoichiometric air-fuel ratio is disclosed in
No. 17732.

[0003]

In this method, when the air-fuel ratio is switched between the lean air-fuel ratio and the stoichiometric air-fuel ratio, NOx is generated in the process of changing the air-fuel ratio between the lean air-fuel ratio and the stoichiometric air-fuel ratio.
Since the amount of emission increases, in order to reduce the amount of NOx emission, it is necessary to increase the air-fuel ratio switching speed and switch quickly. On the other hand, when the torque step at the time of switching from the lean air-fuel ratio to the stoichiometric air-fuel ratio is large, this is felt as a driving shock.In such a case, the switching speed of the air-fuel ratio is reduced so that the driving shock is eliminated, It is desirable to gradually switch to the stoichiometric air-fuel ratio.

However, in the above device, since the switching speed of the air-fuel ratio is a constant value, both cannot be satisfied.

On the other hand, a change speed of the throttle valve opening (a change amount of the throttle valve opening per unit time) representing the acceleration level is detected, and a switching speed of the air-fuel ratio is set according to the change speed. Although there is also (JP-A-60-50241),
Since the load state before the switching of the air-fuel ratio is not considered, it is not possible to cope with all the load states during the lean operation immediately before the switching of the air-fuel ratio.

For example, considering the case where the vehicle is accelerated in an empty state during lean operation and the case where the vehicle accelerates in a loaded state under the same degree of acceleration, the torque associated with the air-fuel ratio switching in the loaded state is greater than in the empty state. Because the step becomes large,
If the air-fuel ratio switching speed is set to a small value so as to reduce the driving shock in the loaded state, the air-fuel ratio switching speed becomes unnecessarily low when the vehicle is in an empty state, and the NOx emission increases accordingly. Conversely, if the air-fuel ratio switching speed is set to be high in accordance with the state of the empty vehicle, a driving shock occurs in the loaded state.

Therefore, the present invention reduces the current engine load.
When starting to switch the air-fuel ratio target value from lean air-fuel ratio to stoichiometric air-fuel ratio
Representing the load of the air-fuel ratio target value before switching the air-fuel ratio switching speed at point
By setting according to the engine load storage value ,
And an object thereof to correspond to all of the load state of before and the air-fuel ratio target value switching.

[0008]

According to a first aspect of the present invention, as shown in FIG. 21, a current engine load is stored in an engine load storage value.
A means 56 for determining whether or not the engine is in a preset lean operation area based on a detection signal of an operating condition; and Means 52 for setting a lean target value, and an air-fuel ratio value from the lean target value to a stoichiometric air-fuel ratio.
Means 53 for setting the air-fuel ratio enrichment change speed Ddmrr according to the engine load storage value at the start of the switching of the standard value, and the air-fuel ratio target value is set to a lean target value by this air-fuel ratio enrichment change speed Ddmrr. a means 54 gradually closer to the stoichiometric air-fuel ratio from and provided a means 55 for performing an air-fuel ratio control based on the air-fuel ratio target values have rather close to this gradually stoichiometric air-fuel ratio. The engine load storage means 56
Calculates the weighted average value of the engine load equivalent value, for example.
In this case, the engine load phase
The weighted average value of this value becomes the engine load stored value.

[0009] The second invention is the first invention, to detect the load change rate at the time of starting the switching of the air-fuel ratio target value to the stoichiometric air-fuel ratio, the end in response to the load change rate
The gin load stored value is corrected.

In a third aspect based on the second aspect, the load change speed is a change speed of a throttle valve opening.

[0011]

According to the first aspect of the present invention, even if the degree of acceleration is the same, the engine representing the load before switching of the air-fuel ratio target value as in the case of an empty vehicle
When the load memory value is small, the air-fuel ratio enrichment change speed D
dmr is large and the air-fuel ratio target value is a lean target value
Since it is quickly switched to Luo stoichiometric air-fuel ratio without emission of NOx increases, also the air-fuel ratio th from the low load region
Because when the target value switching small torque step during switching is,
Even if the air-fuel ratio enrichment change speed is high, the driver does not feel a driving shock.

[0012] On the other hand, the air-fuel ratio target value changeover as loading state
When the engine load memory value representing the previous load is large, the air-fuel ratio enrichment change speed Ddmrr becomes small, and the air-fuel ratio
The target value is switched from the lean target value to the stoichiometric air-fuel ratio at a relatively slow speed. When the load of the air-fuel ratio target value before switching is large, although the torque step involved in the air-fuel ratio target value switching is large, by what a slow air-fuel ratio enrichment changing speed, so that the operating shock does not occur It is made.

[0013] In the second aspect of the present invention, the air-fuel ratio eyes to the stoichiometric air-fuel ratio
Also an engine load stored value representing the load of the before and target value switching are the same, since the air-fuel ratio enrichment changing speed as about acceleration increases according to the degree of acceleration is accelerated, the air-fuel ratio enrichment change speed in accordance with the order of the acceleration is given .

Since the throttle valve that interlocks with the accelerator pedal is the one that responds most responsively to the degree of acceleration in the load, according to the third invention, if the load change speed is the change speed of the throttle valve opening, It is possible to switch to the stoichiometric air-fuel ratio with good response even during rapid acceleration.

[0015]

1, an engine body 1 is provided with a fuel injection valve 7 located downstream of an intake throttle valve 5 in an intake passage 8 thereof. Fuel is injected and supplied into the intake air so that a predetermined air-fuel ratio is obtained. The control unit 2 receives a rotation speed signal from a crank angle sensor 4, an intake air amount signal from an air flow meter 6, an air-fuel ratio (oxygen concentration) signal from an oxygen sensor 3 installed in an exhaust passage 8, and a water temperature sensor 11. The engine cooling water temperature signal, the gear position signal from the gear position sensor 12 of the transmission, and the like are input, and the lean air-fuel ratio and the stoichiometric air-fuel ratio are controlled according to the conditions while determining the operating state based on these.

A three-way catalyst 10 is provided in the exhaust passage 8,
During operation at the stoichiometric air-fuel ratio, reduction of NOx in exhaust gas and oxidation of HC and CO are performed with maximum conversion efficiency. When the three-way catalyst 10 has a lean air-fuel ratio, HC and CO are oxidized, but NOx reduction efficiency is low.

However, the more the air-fuel ratio shifts to the lean side, the smaller the amount of NOx generated. If the air-fuel ratio is higher than a predetermined air-fuel ratio, it can be reduced to the same level as that obtained by purifying with the three-way catalyst 10. Fuel efficiency is improved as the lean air-fuel ratio is reached. On the other hand, when operating at a lean air-fuel ratio, combustion tends to be unstable depending on operating conditions.

Therefore, in this example, in a predetermined operating region where the load is not so large, the operation is performed with the lean air-fuel ratio, the engine stability is detected at the same time, and if the engine stability deteriorates during the lean operation, the air-fuel ratio Is shifted to the rich side to ensure stability, that is, to perform stability feedback control at a lean air-fuel ratio to maintain good fuel economy characteristics without impairing engine stability.

The contents of the control executed by the control unit 2 will be described with reference to the following flowchart.

First, FIG. 2 is a diagram for calculating a fuel-air ratio correction coefficient Dml for performing feedback control to an air-fuel ratio necessary for stabilizing the engine while detecting rotation fluctuation of the engine during operation with a lean air-fuel ratio. , Every 180 crank angles.

First, in step A), an inter-REF cycle Ref is read from the reference signal REF at every 180 degrees of the crank angle sensor 4, and in step B), the rotation fluctuation of the engine is calculated based on this Ref. The operation of calculating the rotation fluctuation is shown in the flowchart of FIG.

In step A) of FIG. 3, the period Refrv of one rotation section of the engine is obtained by the following equation: Refrv = Ref + Ref _n-1 (1) where Ref _n-1 is obtained by the previous equation of Ref. The old value of the rotation speed Nrv is shifted, and the previous data is stored in the RAM twice.
And 3 times before to 4 times before.

Next, in step C), Nrv = KN # / Refrv (2) where KN # is converted to the engine speed Nrev using Refrv in accordance with the following equation: conversion constant from period to speed.

In step D), the rotational speed change amount D for each cylinder
The shift of the old value of nerv is performed in the same manner as the shift of nerv, and the new Dnerv is calculated as follows: Dnerv = Nerv-Nerv _n-4 (3) where Nerv _n-4 ; Is calculated by

In this case, a four-cylinder engine is taken as an example, and the rotational speed change amount Dnerv is the change amount of the current one rotation cycle with respect to the one rotation cycle at the time of the previous combustion of the own cylinder (fourth previous combustion cylinder). Become. It should be noted that the variation is taken for each cylinder in order to prevent the variation between cylinders from being mistaken as a variation.

In step F), Llj, which is the amount of change in the number of revolutions, is calculated by the following formula: Llj = Dnerv-Dnervn _-1 (4) where Dnervn _-1 ;

Here, Llj is the amount of change from the immediately preceding Dnerv to the current Dnerv, and corresponds to a pseudo torque fluctuation accompanying combustion. Then, in step G), the amount of change L
By performing a band-pass filter process on lj and storing the result as a stability signal (rotation fluctuation amount) Lljd, the operation of this flowchart ends.

It should be noted that the band pass filter processing is performed by the EC
U software is available, using the equation converted from continuous system to discrete system,
The frequency may be about 3 to 7 Hz, which is easily felt by the driver of the vehicle as a surge.

After calculating the rotation fluctuation amount in this way,
Returning to FIG. 2, it is determined in step C) whether feedback (F / B) control with a lean air-fuel ratio is performed while monitoring the stability of the engine. This will be described with reference to the flowchart of FIG.

In step A) of FIG. 4, it is determined whether the condition is a lean condition. The lean operation conditions will be described in detail with reference to the flowcharts of FIGS. 7 and 8 described later, which are performed as a background job. Basically, the engine speed and load, and furthermore, the gear position and the vehicle speed are within predetermined ranges, respectively. Done if done. When the lean condition is reached, the control jumps to F / B control inhibition in step L).

However, even under the lean condition, the F / B control is not always performed in order to secure the stability of the control. Therefore, the following items are checked.

In step B), it is determined whether or not the air-fuel ratio is being switched. If Dml obtained in steps G) to K) in FIG. 2 described later is the same as Tdml, it is determined that switching is not being performed. If the switching is being performed, the process jumps to step L) as described above, and the F / B control is prohibited.

Next, it is determined in step C) whether or not the area is in the F / B control area. This is performed by checking a permission flag set according to the rotation speed Ne and the load Tp for the entire operating range of the engine as shown in FIG.
If it is not in the permitted F / B control area, the process jumps to F / B control prohibition. Note that, in this embodiment, the F / B control is performed except in a high rotation range.

In step D), the gear position is checked. If the gear position is less than the predetermined low-speed gear LLGR #, F /
Jump to B control prohibition. This is to inhibit the F / B control when the transmission is in a low-speed gear, because the rotation changes rapidly, for example, at the first speed. Also, when neutral, the F / B control is similarly prohibited. In step F), it is determined whether or not the gear position is being changed by comparing the previous gear position with the current gear position. If it is determined that a gear change has been made, the process also jumps to F / B control inhibition.

Next, in steps G) to I), a determination is made to inhibit the F / B control during the transient operation. The change in the throttle valve opening Tvo, the change in the basic pulse width Tp,
The change amount of the engine speed Ne is set to the set value LLD.
If any of the change amounts exceeds the set value compared to TVO #, LLDNE #, LLDTP #, it is determined that the state is a transient state and the F / B control is prohibited in the same manner as described above.

If all the conditions have been satisfied so far,
In step J), a process of giving a delay of the F / B control is performed. Here, it is checked whether or not a predetermined time TMLLC # has elapsed after all of the steps D) to I) have become the F / B control conditions, and the F / B control is prohibited until the predetermined time has elapsed. For the first time, it is determined that the region is the F / B control region.

The delay is given by the stability signal Llj.
d is passed through a filter, and when it is affected by disturbance, the output is not stabilized immediately, and the rotation fluctuation generated by gear change etc. does not disappear instantaneously due to the vibration system of the vehicle. Yes, more stable F /
In order to perform the B control, it is better to provide a predetermined delay.

After the determination of the F / B control is performed in this manner, the flow returns to FIG. 2 again. In step D), it is checked whether or not the stability F / B control is performed. E) according to the flowchart of FIG.
Correction rate of F / B control, that is, stabilized fuel-air ratio correction coefficient Ll
Update and calculate dml.

Here, a stabilized fuel-air ratio correction coefficient for performing the F / B control is calculated based on the rotation fluctuation amount calculated as described above. First, in step A), the above-mentioned stability signal Lljd is calculated. Sample and count this number of samples.

In step B), the number of samples is set. The number of samples is set as shown in FIG.
The set values of the long L and the short S, which change depending on e, are read in accordance with the rotation speed Ne at that time. In this case, the detection accuracy increases as the number of samples increases, but the control speed decreases (lower). So fast), so it is decided in consideration of these.

Next, it is determined in step C) whether or not the number of samples is equal to S. If they are, step D)
In step E), the average value is obtained by dividing the sample data total by S, and the average result is compared with SLH #, which is a high-speed control stability determination comparison value. In F), the stabilized fuel-air ratio correction coefficient Lldml is calculated as follows: Lldml = Lldml _n-1 + DLLH # (5) where Lldml _n-1 ; Lldml DLLH # immediately before one time; high-speed update amount (predetermined value) of Lldml Update with expression.

When this is completed, or if it is less than SLH #, the process directly proceeds to step G), where the number of samples is compared with L which is larger than S. Is divided by L to obtain an average value, and a value obtained by subtracting the low speed control stability determination comparison value SLL # from the average value is used to obtain an update amount Lldml (+ //) of Lldml from the characteristic map shown in FIG.
-) Is calculated. In this characteristic diagram, in order to prevent torque fluctuation (shock) caused by changing the fuel-air ratio by this control, the dead zone where Dlldml = 0 is defined by the boundary between the region where the update amount is positive and the region where the update amount is negative. And a predetermined width around the center.

Then, in step J), the stabilized fuel / air ratio correction coefficient Lldml is updated by the following equation: Lldml = Lldml _n-1 + Dldml (6)

Although omitted in the flowchart, this Ll
The range of dml is limited so that 1.0 ≦ Lldml ≦ LLDMMX # (where LLDMMMX # is the preset maximum value of the stabilized fuel-air ratio correction coefficient), and this control operation is terminated. Lldml is stored in the memory,
It is constantly updated during F / B control.

Therefore, the stabilized fuel-air ratio correction coefficient Lldm
l becomes a larger value as the rotation fluctuation amount increases, that is, as the stability of the engine deteriorates.

Next, returning to FIG. 2, the F /
After completing the calculation of the correction rate of the B control, the process proceeds to step F) in FIG. 2 to calculate the target fuel-air ratio Tdml.
This target fuel-air ratio is calculated by searching for the fuel-air ratio Mdml set in the characteristic map shown in FIG. 9 or FIG. 10, and correcting this by the stabilized fuel-air ratio correction coefficient Lldml during F / B control. In this case, one of the maps is selected depending on whether or not the vehicle is in the lean operation condition.

Here, the determination of the lean operation condition will be described with reference to the flowcharts of FIGS.

These operations are performed as a background job. Lean conditions are determined in step A) of FIG. 7, and specific contents for this are shown in FIG.
The lean condition is determined by checking the contents of steps A) to F) of FIG. 8 one by one. When all of the items are satisfied, lean operation is permitted. Ban.

That is, Step A): The air-fuel ratio (oxygen) sensor is activated, Step B): The warm-up of the engine is completed, Step C): The load (Tp) is in a predetermined lean region. Step D): The rotational speed (Ne) is in a predetermined lean region. Step E): The gear position is in the second speed or higher. Step F): The vehicle speed is in a predetermined range. The lean operation is permitted, and if not, the process proceeds to step I) to prohibit the lean operation.
The above steps A) to F) are conditions for stably performing the lean operation without impairing the operation performance.

The above-described flow of FIG. 8 constitutes means for determining the lean operation region.

When the lean condition is determined in this way,
Returning to steps C) and D) in FIG. 7, when the condition is not the lean condition, the stoichiometric fuel-air ratio or a map fuel-air ratio deeper than the stoichiometric fuel-air ratio is determined in step C), and the characteristic map shown in FIG. Calculated by searching with Tp,
On the other hand, under the lean condition, the map fuel-air ratio Mdml that is thinner by a predetermined range than the stoichiometric air-fuel ratio in step D)
Is similarly searched according to the characteristic map shown in FIG.

The numerical values shown in these maps are relative values when the stoichiometric air-fuel ratio is 1.0, so that a larger value indicates richer and a smaller value indicates leaner. The above-described flow of FIG. 7 and the map of FIG. 9 constitute means for setting the air-fuel ratio target value in the lean operation region.

Here, returning to step F) of FIG. 2 again,
Of the map fuel-air ratio Mdml calculated in this way, for Mdml under the lean condition, based on the stabilized fuel-air ratio correction coefficient Lldml, Tdml = Mdml × Lldml (7) where Mdml: target fuel-air The target fuel-air ratio Tdml is calculated by correcting with the equation of the map value of the ratio.

The target fuel-air ratio Tdml is such that Lldml increases as the engine rotation fluctuation increases.
As the degree of stability deteriorates, the value increases, that is, the target air-fuel ratio shifts to the rich side.

The following step G) is to gradually change the air-fuel ratio in the damper operation process at the time of switching the fuel-air ratio, thereby preventing a sudden change in the torque and ensuring the stability of the driving performance. .

In step G), the fuel-air ratio correction coefficient Dml is compared with the previously calculated Tdml.
If not ≧ Tdml, that is, if the calculated target fuel-air ratio is larger than the held fuel-air ratio correction coefficient Dml, in order to shift the air-fuel ratio to the rich side in steps H) and I), Dmlr corresponding to the air-fuel ratio change speed to the rich side is added to the correction coefficient Dmln _-1 to obtain a new D
Obtain ml. Then, Dml is limited so that the fuel-air ratio correction coefficient Dml does not exceed the calculated target fuel-air ratio Tdml.

On the other hand, if Dml ≧ Tdml,
In steps J) and K), by subtracting the air-fuel ratio change speed Ddmll toward the lean side from the held Dml,
A new fuel-air ratio correction coefficient Dml shifted to the lean side is obtained, and Dm is adjusted so that Dml does not become less than Tdml.
l is restricted.

Step G) of the flow of FIG.
H) constitutes means for gradually bringing the air-fuel ratio target value closer to the stoichiometric air-fuel ratio at every air-fuel ratio enrichment change speed.

It should be noted that there is no lean condition, and based on the characteristic map shown in FIG.
When dml is calculated, although not shown, the map fuel-air ratio Mdml in step F) is not corrected by the stabilized fuel-air ratio correction coefficient Lldml, and this Md
The fuel-air ratio correction coefficient Dml may be calculated by directly replacing ml with the target fuel-air ratio Tdml in step G).

Based on the fuel-air ratio correction coefficient Dml calculated in this way, the following calculation of the fuel injection amount is performed.

The flowchart of FIG. 6 shows the control operation for calculating and outputting the fuel injection pulse width using the fuel-air ratio correction coefficient Dml obtained in this manner. Using the coefficient Dml, the target fuel-air ratio Tfbya is calculated by the following equation: Tfbya = Dml + Ktw + Kas (8) where Ktw: water temperature increase correction coefficient Kas;

Here, Ktw is a fuel increase according to the cooling water temperature, and Kas is a fuel increase immediately after starting. Next, in step B), the output of the air flow meter is A / D converted and linearized to calculate the intake air flow rate Q. In step C), based on the intake air flow rate Q and the engine speed Ne, the basic pulse width Tp to be given to the fuel injection valve is calculated as Tp =
It is obtained as K × Q / N. K is a constant.

Then, in step D), based on this Tp, one fuel injection pulse width Ti is calculated as follows: Ti = Tp × Tfbya × Ktr × (α + αm) + Ts (9) where Ktr; Correction coefficient α; air-fuel ratio feedback correction coefficient αm; air-fuel ratio learning correction coefficient Ts; invalid pulse width

However, under the lean condition, these K
tr, α, αm, etc. are fixed at predetermined values.

Next, the fuel cut is determined in step F), and the invalid pulse width Ts is stored in the output register if the fuel cut condition is satisfied in steps G) and H), and Ti is stored in the output register otherwise. In preparation for injection at a predetermined injection timing according to the output of

The above-described flow of FIG. 6 constitutes means for performing air-fuel ratio control based on the set air-fuel ratio.

The fuel injection pulse width Ti is calculated in this manner. Therefore, when the stability is deteriorated during the stability feedback control in the lean operation, the set air-fuel ratio is shifted to the rich side accordingly, and Of fuel consumption and NOx without sacrificing drivability
To reduce

By the way, as shown in FIG. 14, in the operation with a lean air-fuel ratio, the leaner the air-fuel ratio, the lower the NOx emission level, but the worse the engine stability. Therefore, if the air-fuel ratio is maintained such that the NOx emission level is lower than the allowable limit and the engine stability is also at the allowable limit, it is possible to sufficiently reduce NOx without impairing the stability of the engine. Becomes
Deterioration of stability can be dealt with by shifting the air-fuel ratio to the rich side.
NOx emission levels exceed acceptable limits. Also,
If the air-fuel ratio blindly shifts to the lean side just because NOx decreases, combustion deteriorates, and stable operation of the engine cannot be maintained.

The A characteristic and the B characteristic in FIG.
The relationship between the NOx emission amount and the engine stability when the air-fuel ratio is changed under the A operating condition (load and rotation) and the B operating condition is shown. Even at the air-fuel ratio of NOx, the NOx emission characteristics and the stability characteristics are different.

Therefore, the map air-fuel ratio set as the target air-fuel ratio at the time of the lean operation as shown in FIG. 9 is used to determine the limit of the rich-side air-fuel ratio and the stability of the stability from the NOx emission amount while considering these operating conditions. If the predetermined value is set within a range from the limit to the limit of the lean air-fuel ratio, the lean operation can be performed under the condition that the NOx and the stability are always in a constant range. In this case, the stabilized fuel-air ratio correction coefficient Ll
Since the dml and the NOx emission amount substantially correspond to each other, the NOx emission amount can always be suppressed within the allowable limit without measuring the NOx emission amount.

The target air-fuel ratio is set, for example, by NO
The air-fuel ratio value is lean from the air-fuel ratio at the emission limit of x by a certain value, the air-fuel ratio value on the rich side by a certain value from the air-fuel ratio at the stability limit, the air-fuel ratio value approximately halfway between both limit air-fuel ratios, and both The limit air-fuel ratio can be set as an air-fuel ratio value that internally divides by a fixed ratio.

When the air-fuel ratio is switched between the lean air-fuel ratio and the stoichiometric air-fuel ratio, the air-fuel ratio is quickly switched by increasing the air-fuel ratio enrichment change speed Ddmrl in order to reduce the amount of NOx emission. On the other hand, when the torque step at the time of switching from the lean air-fuel ratio to the stoichiometric air-fuel ratio is large, this is felt as a driving shock. In this case, the air-fuel ratio enrichment change speed D
It is desirable to reduce dmlr and gradually switch to the stoichiometric air-fuel ratio.

To cope with this, the control unit 2 sets the above-described air-fuel ratio enrichment change speed Ddmrl shown in step H) of FIG. 2 according to the load before switching the air-fuel ratio as shown in FIG. .

In FIG. 15, first, a switching speed setting load Tpl is calculated in step A). This operation is shown in FIG.
6 is shown.

In step A) of FIG. 16, the basic pulse width (load equivalent amount) Tp is read, and in step B), the old value of the weighted average value Avtpl of Tp is shifted, and the data of one time before is compared with the data of two times before. Transfer to RAM and 3 times to 4 times.

In step C), a new Avtpl is calculated as follows: Avtpl = Tp × (1 / N) + Avtpl _n−1 × (1-1 / N) (11) where 1 / N: weighted average coefficient Avtpl _n−1 ; It is calculated by the formula of Avtp1 one time before. Since Tp fluctuates under the influence of the intake pulsation, a weighted average is used to eliminate this fluctuation.

In step D), it is determined whether or not the condition is a lean condition. If the condition is a lean condition, the counter value L is incremented in step E), and the counter value L is incremented by a predetermined value L0.
And step F). This counter measures the time from the start of the lean condition. When L becomes equal to or greater than L0, it is determined that the operation has shifted to the lean operation, the predetermined time has elapsed, and the value of Avtp1 has stabilized. Avt which is the value m times (for example, 3 or 4 times before) in step G)
adopt pl _nm as Tpl.

On the other hand, when L <L0, the process proceeds to step H), and the current value Avtpl is directly used as Tp
adopted as l. This may be returned to the operation at the stoichiometric air-fuel ratio during the switching to the lean operation, and at this time, it is desirable to reflect the state of the lean operation as much as possible. This is because when the operation is returned to the stoichiometric air-fuel ratio, the current value Avtpl becomes the leanest value.

The flow shown in FIG. 16 corresponds to the current engine load.
This constitutes means for storing the load as an engine load storage value .

Thus, the switching speed setting load Tp
After calculating l, returning to FIG. 15, steps B), C),
In D), Tpl and three determination values Tp3, Tp2, T
By comparing p1, it is determined which load region Tpl is in. The three determination values Tp3, Tp2, and Tp1 are used to divide the load range into four broad ranges. Here, Tp3
>Tp2> Tp1 so that Tpl
≧ Tp3 region, Tp3> Tpl ≧ Tp2 region,
The load region becomes smaller in the order of Tp2> Tpl ≧ Tp1 and Tp1> Tpl.

When it is determined which of these load ranges belongs to, in steps E), F), G) and H), the set values (DDMLR0, DDMLR, DDMLR) corresponding to the load range to which the load range belongs.
1, DDMLR2, DDMLR3) is adopted as the air-fuel ratio enrichment change speed Ddmrr, and the processing in FIG. 15 is terminated.

Four setting values DDMLR0, DDMLR
1, the size of DDMLR2 and DDMLR3 is DDMLR0
Since <DDMLR1 <DDMLR2 <DDMLR3, the air-fuel ratio enrichment change speed Ddml decreases as the load decreases.
The value of r is increased. The torque step when switching from the low load state to the stoichiometric air-fuel ratio by performing acceleration is smaller than the torque step when switching from the high load state to the stoichiometric air-fuel ratio by accelerating. This is because the smaller the load, the higher the air-fuel ratio enrichment change speed.

Here, the operation of this example will be described with reference to FIG. 17. FIG. 17 shows waveforms when the air-fuel ratio is switched to the stoichiometric air-fuel ratio by acceleration from the lean operation state.
Even when the change rate ΔTVO of the throttle valve opening indicating the degree of acceleration is the same, when the load immediately before switching of the air-fuel ratio is small, as in the case of an empty vehicle, the air-fuel ratio enrichment change rate Ddmrl increases and the air-fuel ratio can be switched quickly. (See the solid line at the bottom of FIG. 17), the NOx emission amount does not increase, and the torque step at the time of air-fuel ratio switching from a low load range is small, so that even if the air-fuel ratio switching speed is fast, The driver does not feel a driving shock.

On the other hand, when the load immediately before the switching of the air-fuel ratio is large, such as in the loaded state, the air-fuel ratio enrichment change speed Ddm
As r becomes smaller, the air-fuel ratio is switched at a relatively slow speed (see the broken line at the bottom of FIG. 17). When the load immediately before the switching of the air-fuel ratio is large, since the torque step accompanying the switching of the air-fuel ratio is large, the switching shock of the air-fuel ratio is made slow so that the driving shock does not occur.

As described above, the switching speed of the air-fuel ratio when switching from the lean air-fuel ratio to the stoichiometric air-fuel ratio is increased in accordance with the load immediately before the switching of the air-fuel ratio, as the load becomes smaller. When the step is small, the switching time of the air-fuel ratio is shortened to prevent the NOx emission from increasing, and when the torque step at the time of switching the air-fuel ratio is large, the drivability is prioritized so that no driving shock occurs. be able to.

FIG. 18 shows a second embodiment, corresponding to FIG. 18 differs from FIG. 15 in that the change rate ΔTVO of the throttle valve opening during acceleration in step B) is different.
(ΔTVO> 0), and in step C) from this ΔTVO, a table containing the contents of FIG. 18 is searched to find a load correction coefficient KTVO. In step D), the above-described load Tpl for setting the switching speed is calculated as Tpl2 = Tpl × KTVO (12) corrected by the following equation.

Even if the load immediately before switching the air-fuel ratio to the stoichiometric air-fuel ratio is the same, the air-fuel ratio switching speed increases as the degree of acceleration increases in accordance with the degree of acceleration. Therefore, as shown in FIG.
The value of KTVO is increased in proportion to.

In this example, by taking into account ΔTVO, for example, as shown in FIG. 20, when the load immediately before switching the air-fuel ratio is small and ΔTVO is large, ΔTVO is large because the load immediately before switching the air-fuel ratio is large. When large, when the load immediately before the air-fuel ratio switching is small and ΔTVO is small,
The air-fuel ratio enrichment change speed Ddmrl decreases in the order in which the load immediately before the air-fuel ratio switching is large and ΔTVO is small.

In this example, the switching speed of the air-fuel ratio is set in accordance with ΔTVO. As a result, the air-fuel ratio switching speed in accordance with the degree of acceleration can be changed as in the above-mentioned Japanese Patent Application Laid-Open No. 60-50241. Given.

In the embodiment, Tp is used as the load immediately before the switching of the air-fuel ratio. However, the throttle valve opening may be used.

[0091]

According to the first invention, the current engine load is reduced.
Means for storing the engine load storage value, means for determining whether or not the engine is in a predetermined lean operation area based on a detection signal of the operating condition, and means for determining the stoichiometric air-fuel ratio when the lean operation area is determined. means for setting the lean target value than the air-fuel from the lean target value to the stoichiometric air-fuel ratio
When the switching of the ratio target value is started, the engine load memory is stored.
Means for setting an air-fuel ratio enrichment change speed according to the value ;
By the air-fuel ratio enrichment change speed dilute the air target value
Means for gradually approaching the stoichiometric air-fuel ratio from a thin target value ;
It is provided with the means for performing the air-fuel ratio control based on the air-fuel ratio target value rather nearing to the gradually stoichiometric air-fuel ratio
Air-fuel ratio target when torque step when the target value switching is small
While preventing the emission of NOx by shortening the switching time value increases, when torque step during the air-fuel ratio target value switching is large, give priority to the drivability, can be prevented cause operation shock .

[0092] The second invention is the first invention, to detect the load change rate at the time of starting the switching of the air-fuel ratio target value to the stoichiometric air-fuel ratio, the end in response to the load change rate
Since the gin load storage value is corrected, the air-fuel ratio enrichment change speed according to the degree of acceleration can be given.

According to a third aspect of the present invention, in the second aspect, the load change speed is a change speed of the throttle valve opening degree, so that the stoichiometric air-fuel ratio can be switched to the stoichiometric air-fuel ratio with good response even during rapid acceleration.

[Brief description of the drawings]

FIG. 1 is a configuration diagram showing an embodiment of the present invention.

FIG. 2 is a flowchart of a 180-degree job.

FIG. 3 is a flowchart for explaining calculation of rotation fluctuation.

FIG. 4 is a flowchart for explaining determination of a feedback control condition.

FIG. 5 is a flowchart for explaining calculation of a stabilized fuel-air ratio correction coefficient Lldml.

FIG. 6 is a flowchart of a 10 msec job.

FIG. 7 is a flowchart of a background job.

FIG. 8 is a flowchart illustrating the determination of a lean condition.

FIG. 9 is a characteristic diagram showing the contents of a lean map.

FIG. 10 is a characteristic diagram showing the contents of a non-lean map.

FIG. 11 is a region diagram showing both a region in which feedback control is performed and a region in which feedback control is prohibited.

FIG. 12 is a characteristic diagram showing a table content of a predetermined number L of samples.

FIG. 13 is an update amount D of the stabilized fuel-air ratio correction coefficient Lldml.
It is a characteristic diagram which shows the table content of lldml.

FIG. 14 is a characteristic diagram showing a relationship between an air-fuel ratio, a NOx emission amount, and stability.

FIG. 15 is a flowchart for explaining the setting of the air-fuel ratio enrichment change speed Ddmrr.

FIG. 16 is a flowchart for explaining calculation of a switching speed setting load Tpl.

FIG. 17 is a waveform diagram illustrating an operation when the lean air-fuel ratio is switched to the stoichiometric air-fuel ratio by acceleration.

FIG. 18 shows the air-fuel ratio enrichment change speed Ddm of the second embodiment.
It is a flowchart for demonstrating setting of 1r.

FIG. 19 is a characteristic diagram of a load correction coefficient KTVO of the second embodiment.

FIG. 20 is a waveform chart for explaining the operation of the second embodiment when the lean air-fuel ratio is switched to the stoichiometric air-fuel ratio by acceleration.

FIG. 21 is a configuration diagram (claim correspondence diagram) of the first invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Engine main body 2 Control unit 3 Oxygen sensor 4 Crank angle sensor 6 Air flow meter 7 Fuel injection valve 51 Lean operation area determination means 52 Air-fuel ratio target value setting means 53 Air-fuel ratio switching speed setting means 54 Theoretical air-fuel ratio approaching means 55 Air-fuel ratio control means

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int.Cl. ⁷ , DB name) F02D 41/00-41/40 F02D 43/00-45/00 F02D 29/00-29/06

Claims (3)

(57) [Claims] An engine load storage value is used to store a current engine load.
Means for determining whether or not the engine is in a predetermined lean operation region based on the detection signal of the operating condition; and, when the lean operation region is determined, the air-fuel ratio is leaner than the stoichiometric air-fuel ratio. Means for setting the target value; and changing the air-fuel ratio enrichment change speed according to the engine load storage value at the time when the air-fuel ratio target value is switched from the lean target value to the stoichiometric air-fuel ratio. means for setting, dilute the air-fuel ratio target value by the air-fuel ratio enrichment changing speed
Engine, wherein the means to approach gradually to the stoichiometric air-fuel ratio of a thin target value, in that a means for performing the air-fuel ratio control based on the air-fuel ratio target value rather nearing to the gradually stoichiometric air-fuel ratio Air-fuel ratio control device. 2. Switching of the air-fuel ratio target value to the stoichiometric air-fuel ratio is started.
2. The air-fuel ratio control device for an engine according to claim 1, wherein a load change speed at a time when the load is changed is detected, and the engine load storage value is corrected according to the load change speed. 3. The air-fuel ratio control device for an engine according to claim 2, wherein the load change speed is a change speed of a throttle valve opening.