JPH08177578A - Air-fuel ratio control device for engine - Google Patents

Abstract

PURPOSE: To dissolve the failure of operability generated in association with the dispersion of performance during lean operation. CONSTITUTION: When a judging means 51 judges an operation area to be a preset lean operation area on the basis of the detection signal of an operating condition, a setting means 52 sets an air-fuel ratio to the target value leaner than a stoichiometric air-fuel ratio. A computing means 54 computes the correction factor of the set air-fuel ratio in the lean operation area in correspondence with the detection value of stability during lean operation. On the basis of this correction factor, a correcting means 55 corrects the set air-fuel ratio in the lean operation area. When the detection value of stability becomes larger than the failure judgment value, an inhibiting means 57 inhibits lean operation.

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 Prior Art Improving engine fuel economy and at the same time NO
In order to reduce x, the fuel supply amount is controlled so that the air-fuel ratio, which is the ratio of air to fuel, becomes a lean air-fuel ratio that is leaner than the theoretical air-fuel ratio. A method of operating an engine in which the fuel ratio is corrected to the rich side to ensure combustion stability is disclosed in Japanese Patent Laid-Open No. Sho 5
It is proposed by Japanese Patent Publication No. 8-217732.

[0003]

By the way, in the case where the engine performance is significantly deteriorated (hereinafter referred to as "hard failure") by the above-mentioned device, for example, the spark plug is soiled, the deposit is caught in the valve seat, the piston ring is worn, etc. When the lean combustion becomes unstable, the air-fuel ratio is gradually corrected to the rich side.Therefore, even if the air-fuel ratio itself is leaner than the stoichiometric air-fuel ratio, the air-fuel ratio is changed to the rich side. Since NOx increases, the engine may be controlled in the range of the air-fuel ratio where the NOx emission amount becomes very large.

In order to deal with this, the stability of the engine is detected during lean operation, and when the stability exceeds a judgment value, it is judged that there is a hardware failure, and lean operation is stopped to return to the stoichiometric air-fuel ratio. Can be considered. On the other hand, even when there is no hard failure, there is a demand to stop lean operation when the engine becomes unstable due to the instability of the engine due to performance variations such as fuel injection valves and swirl control valves, and In this case as well, when the stability detection value becomes equal to or higher than the determination value, it is determined that there is poor drivability due to performance variation, and the lean operation is stopped.

As described above, if there is only one judgment value with respect to two requests, it becomes difficult to set the judgment value, and it is not possible to cope with both a hardware failure and a performance variation. . For example, if it is only necessary to deal with a hardware system failure, the decision value may be increased, and if only to deal with performance variations, a small decision value may be used.
When a large determination value is set, the stability detection value does not exceed the determination value, so that the drivability failure caused by the performance variation is not eliminated. Conversely, when a small judgment value is set, the stability detection value may exceed the judgment value due to factors other than performance variations such as fluctuations from the road surface. It is returned (the operating range at the lean air-fuel ratio is narrowed), and the effect of improving fuel efficiency does not improve so much.

Therefore, the present invention introduces two judgment values, first compares the stability detection value with the first judgment value, and when the stability detection value becomes equal to or higher than the first judgment value, Corrects the air-fuel ratio to the rich side to eliminate poor drivability due to performance variations, and prohibits lean operation when the stability detection value becomes equal to or higher than the second judgment value, thereby responding to hardware failures The purpose is to let.

[0007]

As shown in FIG. 15, a first aspect of the present invention is a means 51 for determining whether or not a lean operating range is preset based on a detection signal of operating conditions.
And means 52 for setting the air-fuel ratio to a target value leaner than the stoichiometric air-fuel ratio when determining the lean operation region, means 53 for detecting the stability of the engine, and the detected stability value during lean operation. Means 54 for calculating a correction coefficient of the set air-fuel ratio in the lean operation range, means 55 for correcting the set air-fuel ratio in the lean operation area based on the correction coefficient, and the corrected set air-fuel ratio Means 56 for performing air-fuel ratio control based on the above, and means 5 for prohibiting lean operation when the detected value of the stability becomes larger than the failure judgment value.
7 and 7.

As shown in FIG. 16, a second aspect of the invention is a means 51 for determining whether or not a lean operating region is preset based on a detection signal of operating conditions, and an empty condition when the lean operating region is determined. Means 52 for setting the fuel ratio to a target value leaner than the theoretical air-fuel ratio, means 53 for detecting the stability of the engine, and so that the detection value of the stability during lean operation becomes the first judgment value or less. Means 61 for calculating the correction coefficient of the set air-fuel ratio in the lean operation range, means 55 for correcting the set air-fuel ratio in the lean operation range based on this correction coefficient, and based on the corrected set air-fuel ratio Means 56 for performing air-fuel ratio control, and means 57 for prohibiting lean operation when the detected stability value becomes larger than the failure judgment value, which is much larger than the first judgment value.

A third aspect of the invention is the fuel cell system according to the first or second aspect of the invention, when the stability detection value is greater than the failure determination value for a predetermined period or longer, the prohibition means 57 leans. Do not execute driving prohibition.

[0010]

According to the first aspect of the invention, the correction coefficient of the air-fuel ratio is calculated according to the stability of the engine during lean operation, and the correction coefficient becomes larger as the stability becomes worse, and accordingly the target air-fuel ratio is set to the rich side. Transition. As long as the stability does not exceed a certain limit, lean operation is continued, so the range of fuel efficiency improvement by lean operation will be expanded to the maximum.

If the stability detection value exceeds the failure determination value due to a hardware failure, it cannot be dealt with by enriching the air-fuel ratio by the correction coefficient, so operation in a lean air-fuel ratio region above this is prohibited. , Promptly switch to stoichiometric air-fuel ratio. At the stoichiometric air-fuel ratio, the operation of the engine will be stable and the conversion efficiency of NOx in the catalyst will be the best, so NO
Emissions of x are maintained at a predetermined low level. In addition, lean operation is prohibited when the detected stability value exceeds the failure judgment value, so the stoichiometric air-fuel ratio is quickly returned to prevent deterioration of drivability. Since I am checking whether to prohibit lean driving in,
There is no delay in control response.

In the second aspect of the present invention, when the lean combustion becomes unstable due to performance variation, the stability detection value becomes equal to or higher than the first determination value, so the target air-fuel ratio is corrected to the rich side. , The poor drivability is eliminated. At this time, since the failure determination value is much larger than the first determination value, the stability detection value does not exceed this failure determination value, and therefore lean operation is continued.

On the other hand, when a hardware failure occurs, the increment of the stability detection value due to the hardware failure becomes much larger than that due to the performance variation, and the stability detection value exceeds the failure determination value and becomes large. , Lean operation is prohibited. Since lean operation is prohibited and operation can be switched to the stoichiometric air-fuel ratio, even if a hardware failure occurs, the three-way catalyst can function effectively while ensuring engine stability and NOx.
Can be reduced.

As described above, the failure determination value is newly introduced in addition to the first determination value, and when the stability detection value becomes equal to or higher than the first determination value, the air-fuel ratio during lean operation is corrected to the rich side. In addition, when the stability detection value becomes equal to or higher than the failure judgment value, it is restored to the stoichiometric air-fuel ratio operation to prevent deterioration of drivability not only due to performance variations but also when hard failures occur. it can.

Even when no hardware failure has occurred, the stability detection value may exceed the failure determination value at any one time. In such a case, the lean operation is prohibited by the third invention. This avoids unnecessarily reducing lean driving opportunities.

[0016]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, reference numeral 1 is an engine body, a fuel injection valve 7 is provided in an intake passage 8 of the engine downstream of an intake throttle valve 5, and an injection signal from a control unit 2 responds to operating conditions. Fuel is injected and supplied into the intake air so that a predetermined air-fuel ratio is achieved. The control unit 2 includes a rotation speed signal from the crank angle sensor 4, an intake air amount signal from the air flow meter 6, an air-fuel ratio (oxygen concentration) signal from the oxygen sensor 3 installed in the 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, etc. 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 installed in the exhaust passage 8,
When operating at the theoretical air-fuel ratio, NOx in the exhaust gas is reduced and HC and CO are oxidized with maximum conversion efficiency. The three-way catalyst 10 oxidizes HC and CO at a lean air-fuel ratio, but has a low NOx reduction efficiency.

However, as the air-fuel ratio shifts to the lean side, the amount of NOx generated decreases, and at a predetermined air-fuel ratio or higher, the NOx can be reduced to the same level as purification by the three-way catalyst 10, and at the same time, The fuel efficiency is improved as the lean air-fuel ratio is achieved. On the other hand, when operating with a lean air-fuel ratio, combustion tends to become unstable depending on operating conditions.

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

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

First, FIG. 2 shows a calculation of a fuel-air ratio correction coefficient Dml for feedback control to the air-fuel ratio necessary for stabilizing the engine while detecting engine rotation fluctuations during operation with a lean air-fuel ratio. , Every crank angle 180.

First, in step A), an inter-REF cycle REF is read from the reference signal REF for each 180 degrees of the crank angle sensor 4, and in step B) the engine rotational fluctuation 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 cycle Refrv of one revolution of the engine is obtained by: Refrv = Ref + Ref _n-1 (1) where Ref _n-1 ; the previous Ref equation, and in step B) the engine The old value of the rotation speed Nev is shifted, and the previous data is stored in the previous RAM twice.
And again, moving from three times to four times.

Next, in step C), Nerv = KN # / Refrv (2) where KN # is used to convert the engine speed Nrev using Refrv according to the equation: conversion constant from cycle to speed.

In step D), the rotational speed change amount D for each cylinder
The shift of the old value of nev is performed in the same manner as the shift of Nev, and here, a new Dnerv is given as: Dnerv = Nev-Nerv _n-4 (3) where Nerv _n-4 ; Calculate with.

In this case, a four-cylinder engine is taken as an example, and the rotation speed change amount Dnerv is the change amount of one rotation cycle at this time with respect to one rotation cycle at the time of combustion of the previous own cylinder (combustion cylinder before four times). Become. The reason why the variation amount is taken for each cylinder is to prevent the variation between the cylinders from being mistakenly recognized as a variation.

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

Here, Llj is the amount of change from the immediately preceding Dnerv to the present Dnerv, and corresponds to a pseudo torque fluctuation associated with combustion. Then, in step G) the change amount L
The bandpass filter process is performed on lj, and the result is stored as the stability signal (rotational fluctuation amount) Lljd, and the operation of this flowchart ends.

The bandpass filter processing is performed by EC
With U software, using the formula converted from continuous system to discrete system,
A frequency of about 3 to 7 Hz may be set so that the driver of the vehicle can easily feel the surge.

The above flow of FIG. 3 constitutes means for detecting the stability of the engine.

When the rotation fluctuation amount is calculated in this way,
Returning to FIG. 2, in step C), it is determined whether to perform feedback (F / B) control with a lean air-fuel ratio while checking 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 lean condition is satisfied. The lean operating condition will be described in detail with reference to the flowcharts of FIGS. 7 and 8 which will be described later, which is performed as a background job. Basically, the engine speed and the load, the gear position, and the vehicle speed are in predetermined ranges. If done. If the lean condition is reached, the F / B control of step L) is skipped.

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

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

Next, in step C), it is judged whether or not it is in the F / B control area. This is done by observing the permission flag set according to the rotation speed Ne and Tp as the load in the entire operating range of the engine, as shown in FIG.
If it is not in the permitted F / B control area, jump to F / B control prohibition. It should be noted that in this embodiment, the F / B control is performed except in the 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 /
Fly to B control ban. This is because the F / B control is prohibited when the transmission is in a low speed gear and the change in rotation is fast. For example, it is prohibited in the first speed. Also, the F / B control is similarly prohibited when in neutral. 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 the gear position is changed, the F / B control is also prohibited.

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

If all the conditions are satisfied so far,
In step J), processing for giving a delay of F / B control is performed. Here, it is checked whether or not a predetermined time TMLLC # has passed after all the steps D) to I) have become F / B control conditions, and the F / B control is prohibited until the time passes, and when the time has passed, For the first time, the F / B control area is determined.

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

When the F / B control is determined in this way, returning to FIG. 2 again, it is checked in step D) whether or not the stability is F / B control, and if it is the F / B control determination, the step is performed. According to the flowchart of FIG. 5 in E),
F / B control correction factor, that is, stabilized fuel-air ratio correction coefficient Ll
Update and calculate dml.

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

The number of samples is set in step B). This is done as shown in FIG.
The set value of long L that changes depending on e is set to N at that time.
The reading is performed in accordance with e. In that case, the detection accuracy increases as the number of samples increases, but the control speed decreases (the smaller the speed, the faster).

Next, in step C), it is judged whether or not the number of samples is L, and if they are, step D).
The sample data total is divided by L to obtain an average value, and the stability determination comparison value SLL # is subtracted from this average value to obtain the update amount Dlldml (+/-) of Lldml from the characteristic map shown in FIG. To calculate. In this characteristic diagram, in order to prevent torque fluctuation (shock) due to changing the fuel-air ratio by this control, the dead zone where Dlldml = 0 is defined as the boundary between the region where the update amount is positive and the region where the update amount is negative. Is provided with a predetermined width around the center.

Then, in step F), the stabilized fuel-air ratio correction coefficient Lldml is updated by the following equation: Lldml = Lldml _n-1 + Dlldml (5) where Lldml _n-1 ;

Therefore, the stabilized fuel-air ratio correction coefficient Lldm
The larger l is, the larger the rotational fluctuation amount is, that is, the worse the stability of the engine is.

Step A) in the flow of FIG. 5 above.
To F) constitute means for calculating the correction coefficient of the set air-fuel ratio in the lean operation region.

In step G), the rich side limit value and the lean side limit value for Lldml are set according to the rotational speed Ne and the basic pulse width Tp as the load at that time. For example, assuming that the limit width to the rich side is LLDMLR (> 0) and the limit width to the lean side is LLDMLL (≧ 0) with 1.0 as the center, these limit widths LLDM
A map having Ne and Tp as parameters for LR and LLDMLL is created in advance, and L is calculated from Ne and Tp at that time.
Search the LDMLR and LLDMLL maps.

At this time, 1.0 + LLDMLR becomes the rich side limit value and 1.0-LLDMLL becomes the lean side limit value. Therefore, in step H), Lldml is compared with the rich side limit value, and Lldml is the rich side limit value. If it exceeds, Lldml is limited to the rich side limit value in step I). Similarly, in step J), the Lldml and lean side limit values are compared, and Lldml is compared.
Is below the lean limit, step K)
Limit Lldml to the lean limit value. In addition,
Lldml is stored in the memory and is constantly updated during F / B control.

The characteristics of the LLDMLR and LLDMLL tend not to be uniform with respect to Ne and Tp. This is because the limit air-fuel ratio for NOx emissions and the limit air-fuel ratio for stability basically depend on the combustion chamber shape, the intake pipe shape, the engine hardware configuration such as the swirl control valve, and the combustion characteristics during lean operation, during lean operation. Depending on the set air-fuel ratio (that is, lean map characteristics) of,
This is because the values of LLDMLR and LLDMLL are determined by the relationship between the different NOx emission limit air-fuel ratio and the stability limit air-fuel ratio. Therefore, after determining the engine hardware configuration and the set air-fuel ratio during lean operation, the characteristics of LLDMLR and LLDMLL are determined by matching.

Further, in order to give priority to the stability of the engine, LLDMLR is made larger than LLDMLL under all conditions of Ne and Tp. As shown in FIG. 14, this is because the rise of the stability in the vicinity of the stability limit air-fuel ratio is tight (that is, the drivability at the stability limit rapidly deteriorates with respect to the change of the air-fuel ratio). This is because the side limit value should be as small as possible.

The steps after step L) will be described in detail later.

Next, returning to FIG. 2 again, in this way F /
After finishing the calculation of the correction rate of the B control, the process proceeds to step F) of FIG. 2 to calculate the target fuel-air ratio Tdml.
This target fuel-air ratio is calculated by searching the fuel-air ratio Mdml set in the characteristic map shown in FIG. 9 or FIG. 10, and correcting it by the stabilized fuel-air ratio correction coefficient Lldml during F / B control. Therefore, in this case, one of the maps is selected depending on whether the operating condition is lean.

Here, the determination of the lean operating condition will be described with reference to the flowcharts of FIGS. 7 and 8.

These operations are performed as a background job, and the lean condition is determined in step A) of FIG. 7. The specific contents for this purpose 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, the lean operation is permitted, and when any of them is not satisfied, the lean operation is performed. Prohibit

That is, step A): the air-fuel ratio (oxygen) sensor is activated, step B): the engine has finished warming up, step C): the load (Tp) is in a predetermined lean range. , Step D): The rotation speed (Ne) is in a predetermined lean range, Step E): The gear position is in the second speed or higher, Step F): The vehicle speed is in a predetermined range, and sometimes, in Step H). If lean operation is permitted, otherwise go to step I) and prohibit lean operation.
The above steps A) to F) are conditions for performing stable lean operation without impairing the operation performance. Step G) not described will be described later.

In the above flow of FIG. 8, step G)
The rest except for constitutes the 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 lean condition is not satisfied, the theoretical fuel air ratio or a map fuel air ratio that is denser than that is calculated in step C), and the characteristic map shown in FIG. Calculated by searching with Tp,
On the other hand, under the lean condition, in step D) the map fuel-air ratio Mdml which is thinner than the stoichiometric air-fuel ratio by a predetermined range.
Are similarly searched according to the characteristic map shown in FIG.

Since the numerical values shown in these maps are relative values with the stoichiometric air-fuel ratio being 1.0, richer values and lean values are indicated. 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.

Now, returning to step F) of FIG. 2 again,
Of the map fuel-air ratio Mdml calculated in this way, for Mdml under lean conditions, 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 the ratio map value.

Since the target fuel-air ratio Tdml becomes larger as the engine speed fluctuation becomes larger, Lldml becomes larger.
It becomes larger as the stability deteriorates, that is, the target air-fuel ratio is shifted to the rich side.

The subsequent step G) and subsequent steps are for the operation of the damper operation at the time of switching the fuel-air ratio to prevent sudden changes in torque by gently switching the air-fuel ratio and to ensure stability of operating performance. .

In step G), the fuel-air ratio correction coefficient Dml is compared with the Tdml calculated above, and if Dml
When ≧ Tdml, that is, when the calculated target fuel-air ratio is larger than the held fuel-air ratio correction coefficient Dml, the air-fuel ratio is shifted to the rich side in steps H) and I). The correction coefficient Dml _n-1 is added to Dmlr corresponding to the air-fuel ratio change speed toward the rich side to obtain a new D
ask for ml. Then, the fuel-air ratio correction coefficient Dml is restricted so that it 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 lean air-fuel ratio change speed Ddmll from the held Dml,
Obtain a new fuel-air ratio correction coefficient Dml that has shifted to the lean side, and make sure that Dml does not fall below Tdml.
Limit l.

The above flow of FIG. 2 constitutes means for correcting the set air-fuel ratio in the lean operation region.

It should be noted that, instead of the lean condition, from the characteristic map shown in FIG.
Although not shown, when the dml is calculated, the map fuel-air ratio Mdml in step F) is not corrected by the stabilized fuel-air ratio correction coefficient Lldml.
The fuel-air ratio correction coefficient Dml may be calculated by replacing ml with the target fuel-air ratio Tdml in step G).

The fuel injection amount correction coefficient Dml thus calculated is used to calculate the fuel injection amount described below.

The flowchart of FIG. 6 shows the control operation contents for calculating and outputting the fuel injection pulse width using the fuel-air ratio correction coefficient Dml thus obtained. First, in step A), the fuel-air ratio correction is performed. Using the coefficient Dml, the target fuel-air ratio Tfbya is calculated by the following formula: Tfbya = Dml + Ktw + Kas (8) where Ktw: water temperature increase correction coefficient Kas: post-start increase correction coefficient.

Here, Ktw is the amount of fuel increase corresponding to the cooling water temperature, and Kas is the amount of fuel increase immediately after the start. 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. Then, in step C), the basic pulse width Tp given to the fuel injection valve is calculated from the intake air flow rate Q and the engine speed Ne as Tp =
Calculate as K × Q / N. Note that K is a constant.

Then, in step D), based on this Tp, the one-time 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 to predetermined values.

Next, in step F), it is determined whether or not the fuel is cut, and in steps G) and H), if the fuel cut condition is satisfied, the invalid pulse width Ts is stored in the output register. Prepare for injection at a predetermined injection timing according to the output of.

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

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

By the way, as shown in FIG. 14, in the operation with the lean air-fuel ratio, the leaner the NOx emission level is, the more the engine becomes lean, but the engine stability also deteriorates. Therefore, if the air-fuel ratio is maintained so that the emission level of NOx is lower than the allowable limit and the stability of the engine is also within the allowable limit, it is possible to sufficiently reduce NOx without impairing the stability of the engine. Becomes
The deterioration of stability can be dealt with by shifting the air-fuel ratio to the rich side, but if it shifts to the rich side too much,
The emission level of NOx exceeds the allowable limit. Also,
If the air-fuel ratio blindly shifts to the lean side because NOx decreases, combustion deteriorates and stable operation of the engine cannot be maintained.

The characteristics A and B in FIG. 14 are shown in FIG.
The relation 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 below. Even if the air-fuel ratio is NOx, the NOx emission characteristics and the stability characteristics are different.

Therefore, the map air-fuel ratio set as shown in FIG. 9 as the target air-fuel ratio during lean operation is taken into consideration, considering these operating conditions, from the NOx emission amount to the rich side air-fuel ratio limit and stability. By setting a predetermined value within the range from the limit to the limit of the lean side air-fuel ratio, lean operation can be performed under the condition that NOx and 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 by, for example, NO.
The air-fuel ratio value that is lean from the air-fuel ratio at the emission limit of x, 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 that is approximately the middle of both limit air-fuel ratios, and both The limit air-fuel ratio can be set as an air-fuel ratio value that internally divides it at a constant ratio.

When a hard failure occurs, lean combustion becomes unstable to a large extent. Therefore, when the stability signal average value exceeds a predetermined judgment value, it is determined that the hard failure has occurred, and the lean operation is performed. It is conceivable to stop the operation and return to the stoichiometric air-fuel ratio. On the other hand, there is also a demand to stop lean operation when the drivability becomes poor due to instability of the engine due to performance variations. If there is only one judgment value for such two requests, it becomes difficult to set the judgment value, and it is not possible to deal with both cases of hardware failure and cases of performance variation.

In order to cope with this, as described above, first, the stability signal average value is compared with the first determination value SLL #, and when the average value is equal to or higher than SLL #, the air-fuel ratio is made rich. It is corrected to the side to eliminate the poor drivability due to performance variations.

Furthermore, a failure judgment value different from the first judgment value is newly introduced, and in the control unit 2,
When the stability signal average value exceeds the failure judgment value, lean operation is prohibited and the operation is switched to the stoichiometric air-fuel ratio.

Specifically, in step L) in FIG. 5, the stability signal average value (= total sample data / L) is compared with the failure determination comparison value SLfail #. This SLfail # is set to a value that can occur only when there is a hardware failure. On the other hand, the first judgment value SL in step E) of FIG.
L # is set to a small value corresponding to the allowable limit of drivability due to performance variations.

As a result of the comparison in step L), when the stability signal average value is SLfail # or more, it is judged that there is a possibility of hardware failure, and the process proceeds to step M), and the timer value Tfail
Is incremented. The timer value Tfail is for measuring the time during which the abnormal state continues, and this timer value Tfail is compared with the predetermined value TFAIL in step N). If you proceed to step M) for the first time, Tfa
Since il is less than TFAIL, the flow of FIG.

Thereafter, the air-fuel ratio control is performed using the stabilized fuel-air ratio correction coefficient Lldml updated in FIG. 5 as described above, and the rotation fluctuation is calculated again. If the stability signal average value calculated from this rotation fluctuation is still SLfail # or more in step L) of FIG. 5, the process proceeds to step M) to increment the timer value Tfail. When the stability signal average value continues to be SLfail # or more, the timer value Tfail increases and eventually, when the timer value Tfail becomes equal to or more than the predetermined value TFAIL in step N), it is determined that a hardware failure occurs, and the procedure goes to step P). Then, the lean prohibit flag Ffail is set. The set state of the flag Ffail is stored in the memory until the engine is stopped. The flag Ffail is reset when the engine is started. On the other hand, even if the stability signal average value is SLfail # or more at all times, the timer value Tfail is TF
When the stability signal average value becomes less than SLfail # before becoming equal to or higher than AIL, the timer value Tfail is cleared in step O) (Tfail = 0). Stability signal average value is SLfail
The reason why a hard failure is judged to be continued for a predetermined time in a state of # or more is to exclude this case because the stability signal average value may be SLfail # or more at any time even though it is not a hard failure. is there.

On the other hand, the above flag Ffa stored in the memory
il is used in step G) of FIG. 8 and flag Ffa
Lean operation is prohibited when il is set.
This prohibition switches to operation at the stoichiometric air-fuel ratio.

The above step G) of FIG. 8 and steps L) to P) of the flow of FIG. 5 constitute means for prohibiting lean operation when the detected stability value becomes equal to or higher than the failure judgment value. ing.

The operation of this example will be described below.

When lean combustion becomes unstable due to performance variation, the stability signal average value becomes equal to or higher than the first judgment value SLL #, so that the stabilized fuel-air ratio correction coefficient Lldml is increased. Since it is updated and the air-fuel ratio is corrected to the rich side, poor drivability is eliminated.

At this time, since the failure judgment value SLfail # is much larger than the first judgment value SLL #, the failure judgment value SLfail # does not exceed the stability signal average value. Lean operation is continued.

On the other hand, when a hard failure occurs, the increment of the stability signal average value due to the hard failure becomes much larger than that due to the performance variation, and the stability signal average value becomes equal to or higher than the failure judgment value SLfail #. Therefore, the flag F
The faif is set and lean operation is prohibited. Since the lean operation is prohibited and the operation is switched to the stoichiometric air-fuel ratio, the three-way catalyst 10 can be effectively operated and NOx can be reduced while ensuring the stability of the engine even in the case of a hardware failure.

As described above, the failure determination value is newly introduced in addition to the first determination value, and when the stability signal average value becomes equal to or higher than the first determination value SLL #, the air-fuel ratio during lean operation is increased. Is corrected to the rich side, and when the stability signal average value becomes equal to or higher than the failure determination value SLfail #, the operation is returned to the stoichiometric air-fuel ratio operation, and the lean operation is prohibited until the engine is stopped thereafter. It is possible to prevent deterioration of drivability not only in the case of, but also when a hardware failure occurs.

Even when no hardware failure has occurred,
In some cases, the average stability signal may exceed the failure determination value. In such a case, the flag Ffa
Since il is never set (that is, a false diagnosis of a hard failure is avoided), unnecessary lean operation opportunities are avoided.

In the embodiment, the example in which the stabilized fuel-air ratio correction coefficient is calculated so that the stability becomes the target value has been described, but the stabilized fuel-air ratio correction coefficient is calculated so that the stability becomes the target value or less. It doesn't matter what you do. An example in which the stability is equal to or less than the target value is one in which Lldml is not updated to the lean side in steps E) and F) of FIG. 5 (that is, Dlldml = 0 in the region where the average value −SLL # is negative in FIG. 13). )
Is.

[0093]

According to the first aspect of the present invention, the air-fuel ratio is corrected according to the stability of the engine, and when the stability becomes equal to or higher than the failure determination value, the air-fuel ratio is switched to the stoichiometric air-fuel ratio without leaning. As long as the stability does not exceed a certain limit, lean operation can be continued to maximize the range of fuel efficiency improvement by lean operation, and at the time of hard failure, engine operation can be stabilized by operating at the theoretical air-fuel ratio. , NOx
Emissions will not increase. Further, when the stability exceeds the limit, the stoichiometric air-fuel ratio is promptly returned, so that it is possible to prevent deterioration of drivability. Further, since it is determined whether lean operation is prohibited by the stability itself, there is no control response delay.

A second aspect of the invention is a means for judging whether or not it is in a lean operating range set in advance on the basis of a detection signal of operating conditions, and an air-fuel ratio leaner than the theoretical air-fuel ratio when the lean operating range is judged. Means for setting a desired target value, a means for detecting the stability of the engine, and a set air-fuel ratio in the lean operation region so that the detected value of the stability during the lean operation becomes the first judgment value or less. A means for calculating a correction coefficient, a means for correcting the set air-fuel ratio in the lean operation region based on the correction coefficient, a means for performing air-fuel ratio control based on the corrected set air-fuel ratio, and the stability And a means for prohibiting the lean operation when the detected value becomes larger than the failure determination value, which is much larger than the first determination value, the lean operation is continued for the performance variation. As well as eliminate the driveability poor due to variations, the operation of the engine can be stabilized by the operation at the stoichiometric air-fuel ratio at the time of hardware failure, never receive more emissions of NOx.

A third aspect of the invention is the fuel cell system according to the first or second aspect of the invention, when the stability detection value is greater than the failure determination value for a predetermined period or longer, the lean operation by the prohibiting means is performed. Since the prohibition is not executed, it is possible to prevent erroneous diagnosis of a hard failure even if no hard failure has occurred, and to avoid unnecessarily reducing the opportunity for lean operation.

[Brief description of drawings]

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

FIG. 2 is a flow chart 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 feedback control conditions.

FIG. 5 is a flow chart 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 for explaining lean condition determination.

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

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

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

FIG. 12 is a characteristic diagram showing the contents of a table of a predetermined sample number L.

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

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

FIG. 15 is a configuration diagram (claims correspondence diagram) of the first invention.

FIG. 16 is a configuration diagram (claims correspondence diagram) of the second invention.

[Explanation 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 Operating Region Determining Means 52 Air-Fuel Ratio Target Value Setting Means 53 Stability Detecting Means 54 Correction Coefficient Calculating Means 55 Set Air-Fuel Ratio Correcting Means 56 Air-fuel ratio control means 57 Lean operation prohibition means 61 Correction coefficient calculation means

Claims (3)

[Claims] Claim: What is claimed is: 1. Means for determining whether a lean operating range is preset based on an operating condition detection signal, and an air-fuel ratio set to a target value leaner than a theoretical air-fuel ratio when the lean operating range is determined. A means for setting, a means for detecting the stability of the engine, a means for calculating a correction coefficient of the set air-fuel ratio in the lean operation region in correspondence with the detected value of the stability during lean operation, and based on this correction coefficient Means for correcting the set air-fuel ratio in the lean operation region, means for performing air-fuel ratio control based on the corrected set air-fuel ratio, and when the detected stability value is greater than the failure determination value Is a unit for prohibiting lean operation, and is an air-fuel ratio control device for an engine. 2. Means for determining whether or not a lean operation region is preset based on an operating condition detection signal, and an air-fuel ratio set to a target value leaner than a theoretical air-fuel ratio when the lean operation region is determined. A means for setting, a means for detecting the stability of the engine, and a correction coefficient for the set air-fuel ratio in the lean operation region so that the detected value of the stability during the lean operation becomes the first judgment value or less. Means for correcting the set air-fuel ratio in the lean operation region based on the correction coefficient, means for performing air-fuel ratio control based on the corrected set air-fuel ratio, and the stability detection value An air-fuel ratio control device for an engine, comprising: means for prohibiting lean operation when the value becomes larger than a failure judgment value, which is much larger than the first judgment value. 3. A lean operation prohibition is not executed by the prohibiting means when a state in which the detected value of the stability becomes larger than the failure judgment value does not continue for a predetermined period or longer. Alternatively, the air-fuel ratio control device for the engine according to item 2.