JPH05141298A - Air fuel ratio control method for gas engine - Google Patents

Abstract

PURPOSE:To prevent deterioration in emission by raising a threshold level of main feedback corresponding to a degree of deterioration of a catalytic converter rhodium in those which control air fuel ratio based on outputs of oxygen sensors provided upstream and downstream of a catalytic converter rhodium. CONSTITUTION:A main sensor 12 and a sub sensor 13 are provided upstream and downstream of a catalytic converter rhodium 11. Those outputs are put into a control device 15 with other sensor outputs. Here, main feedback constants such as a delay time TDL and rich and lean delay time TDR are calculated by a control constant calculating means 20 from the output of the sub sensor 13 and sent to an air fuel ratio correction amount calculating means 21, the air fuel ratio correction amount is calculated based on the output of the main sensor 12 and sent to a bypass gas amount calculating means 17. At this time, when deterioration of the catalytic converter rhodium is judged by a catalyst deterioration judging means 19 from the output of the sub sensor 13, a threshold level of the main sensor 12 is raised by a predetermined amount according to the deterioration degree.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a three-way catalyst arranged in an exhaust passage for purifying exhaust gas, a first oxygen sensor arranged upstream of the three-way catalyst, and the three-way catalyst. In response to the outputs of the second oxygen sensor disposed on the downstream side of the engine and the first and second oxygen sensors, the air-fuel ratio of the gas engine is changed from the theoretical air-fuel ratio from the oxygen concentration in the exhaust gas. An air-fuel ratio control device for detecting whether the vehicle is in a rich state or a lean state and controlling the air-fuel ratio of the gas engine based on the result of the detection. More specifically,
The present invention relates to an air-fuel ratio control method for such a gas engine.

[0002]

2. Description of the Related Art A mode of air-fuel ratio control performed in a conventional gas engine will be described with reference to FIG.

In an intake passage 1 of a gas engine shown in FIG. 1, a mixer 4 for mixing fuel (for example, city gas 13A) from a fuel passage 2 and air from an air intake 3 and a throttle valve 5 are provided in an intake passage 1. And a pressure sensor 6 for detecting the intake pressure. Further, a bypass passage 7 that connects the upstream side of the throttle valve 5 of the intake passage 1 and the fuel passage 2 by bypassing the mixer 4 is provided, and a valve for adjusting the bypass flow rate, that is, an empty passage, is provided in the bypass passage. A fuel ratio control valve 8 is installed.

Here, the gas engine E is provided with a rotation speed sensor 9 for detecting the engine rotation speed.

On the other hand, a three-way catalyst 11 is provided in the exhaust passage 10 of the gas engine 4, and a first catalyst is provided upstream of the catalyst 11.
Oxygen sensor (hereinafter referred to as the main oxygen sensor) 12 is provided, and a second oxygen sensor (hereinafter referred to as the sub oxygen sensor) 13 is provided downstream of the catalyst 11. In addition,
In the illustrated example, the temperature measuring sensor 14 is also provided on the downstream side of the catalyst 11.

Detection signals from the pressure sensor 6, the rotation speed sensor 9, the main and sub oxygen sensors 12, 13, and the temperature measuring sensor 14 are input to an electronic control unit (ECU) 15.
After a predetermined process is performed by the control device 15, it is output to the air-fuel ratio control valve 8 as a control signal.

In the control device 15, the output signals of the pressure sensor 6 and the rotation speed sensor 9 are used as the total gas consumption calculating means 16
The total gas consumption amount TG that is input to and calculated there is sent to the bypass gas amount calculation means 17. The bypass gas amount calculation means 17 calculates, from the bypass ratio table 18, the bypass ratio BR required for operating the engine near the stoichiometric air-fuel ratio.

The output V2 of the sub oxygen sensor 13 is input to the catalyst deterioration determining means 19 and the control constant calculating means 20. Then, the control constant calculation means 20 uses the sub oxygen sensor 1
3 in response to the output V2 of the delay time TDL, TD
A main feedback constant such as R (lean and rich delay time) is calculated and sent to the air-fuel ratio correction amount calculation means 21. Here, the air-fuel ratio correction amount calculation means 21 calculates the air-fuel ratio correction amount FAF based on the delay times TDL, TDR calculated by the control constant calculation means 20 and the output V1 from the main oxygen sensor 12, and the correction amount. The FAF is sent to the bypass gas amount calculation means 17.

The bypass gas amount calculating means 17 calculates the total gas consumption amount TG calculated by the total gas consumption calculating means 16, the bypass ratio BR calculated from the bypass ratio table 18, and the air-fuel ratio correction amount calculating means 21. The bypass gas amount BG is calculated by the following equation (1) based on the air-fuel ratio correction amount FAF calculated by

BG = TG × (BR + FAF) (1) Here, the air-fuel ratio correction amount FAF is + or − with zero as the center.
Is a variable that should vibrate to. Therefore, the air-fuel ratio correction amount F
When the AF has an offset value and vibrates in the area of + or −, the bypass ratio calculation unit 22 changes the bypass ratio BR so as to vibrate around zero, and the bypass ratio table 18 is updated. ..

The bypass gas amount BG calculated by the bypass gas amount calculating means 17 is sent to the air-fuel ratio adjusting means 23. Then, the air-fuel ratio adjusting means 23 controls the bypass gas amount BG.
The opening degree of the air-fuel ratio control valve 8 is set on the basis of the calculation result of the above, and the opening degree of the valve 8 is controlled.

[0012]

Here, the threshold level using a new catalyst, that is, the potential level at which NOx cannot be purified when the potential becomes lower than that potential, is experimentally obtained, and the threshold level is empty. Used in the main feedback of fuel ratio control. In FIG. 7, the characteristic of the output of the main oxygen sensor 12 and the air-fuel ratio λ is shown, and the threshold level of the main feedback is obtained from the characteristic at λ = 1.0, and is indicated by the reference numeral VR1. There is.

However, when the three-way catalyst deteriorates, NOx, C
Since the air-fuel ratio at which the simultaneous maximum purification rate of O is obtained shifts to the rich side, as is apparent from FIG. 7, the output of the main oxygen sensor 12 causes a relative lean shift when the three-way catalyst 11 deteriorates. .. As a result, the main feedback causes a so-called "lean shift", and if the delay time TDR1 is increased by the sub-feedback in order to correct this shift, the air-fuel ratio control vibrates greatly asymmetrically toward the rich side, and the period of the vibration Become slow. As a result, the emission deteriorates and the removal rate of harmful substances decreases.

Here, in the stationary gas engine,
Since the purification rate is required to be a very high value (95% to 99%) as compared with that of a normal internal combustion engine, it is necessary to prevent the deterioration of emission due to the deterioration of the three-way catalyst as described above. .. Replacing the three-way catalyst at an early stage of deterioration is one of the preventive measures against deterioration of emission.
However, since the three-way catalyst itself contains a noble metal and is very expensive, there is the inconvenience that the running cost of the gas engine rises too much, which cannot be said to be a practical countermeasure. That is, there is a demand to use the three-way catalyst as long as possible when operating the gas engine and to keep the cost of the three-way catalyst replacement work low.

In other words, it has been conventionally demanded to provide a technique capable of preventing the deterioration of the emission due to the deterioration of the three-way catalyst and using the three-way catalyst as long as possible to reduce the cost due to the replacement. , The one that responds to it has not been proposed yet.

The present invention has been proposed in view of the above demands, and it is possible to prevent air-fuel ratio control of a gas engine that can prevent deterioration of emission due to deterioration of the three-way catalyst without replacing the three-way catalyst. The purpose is to provide a method.

[0017]

A method for controlling an air-fuel ratio of a gas engine according to the present invention comprises a three-way catalyst arranged in an exhaust passage to purify exhaust gas, and a three-way catalyst disposed upstream of the three-way catalyst. In response to the outputs of the first oxygen sensor (main oxygen sensor), the second oxygen sensor (sub oxygen sensor) disposed downstream of the three-way catalyst, and the first and second oxygen sensors. , Detecting whether the air-fuel ratio of the gas engine is rich or lean to the stoichiometric air-fuel ratio from the oxygen concentration in the exhaust gas, and based on the result, controls the air-fuel ratio of the gas engine And a step of increasing the threshold level of the main feedback in accordance with the degree of deterioration of the three-way catalyst.

In carrying out the present invention, the deterioration of the three-way catalyst is preferably judged by the output amplitude of the sub oxygen sensor. In the main feedback threshold level increasing process, the value obtained by adding the output amplitude of the sub oxygen sensor by a constant k times to the threshold level is set as a new threshold level. Is preferred.

[0019]

According to the present invention having the above-mentioned structure, when the three-way catalyst deteriorates and leans, a main feedback is performed up to the potential corresponding to the lean window based on the characteristics of the main oxygen sensor. Raise the threshold level of. As a result, as shown in FIG. 7, the amplitude at the time of control becomes small and the period becomes short, so that the emission is improved.

Therefore, since the emission can be improved without replacing the three-way catalyst, the life of the expensive three-way catalyst can be extended. As a result, the cost required to replace the three-way catalyst can be saved, and the overall running cost can be kept low.

[0021]

Embodiments of the present invention will be described below with reference to the accompanying drawings.

First, the main feedback in the air-fuel ratio control of the gas engine E shown in FIG. 1 will be described with reference to FIGS.

In FIG. 2, reference numeral V1 in (A) indicates the output of the main oxygen sensor 12, and reference numeral VR1 indicates the first comparison voltage (for example, 0.45 V). When the main oxygen sensor output V1 is larger than the comparison voltage VR1, the rich state is determined, and when the main oxygen sensor output V1 is smaller than the comparison voltage VR1, the lean state is determined. This is shown in FIG. 2 (B).

As described above, in the present invention, the threshold level in the main feedback, that is, the first comparison voltage VR1 rises as the three-way catalyst 11 deteriorates. Details of the rise of the first comparison voltage (main feedback threshold level) will be described later.

FIG. 3 is a flow chart of main feedback for calculating the opening degree FAF of the air-fuel ratio control valve 8 based on the output V1 of the main oxygen sensor 12 in the air-fuel ratio correction amount calculating means 21 (FIG. 1). .. In addition,
In the illustrated embodiment, the control routine of FIG. 3 is executed every 4 ms, for example.

First, in step S1, it is determined whether or not the main feedback condition is satisfied. Here, the main feedback condition is that the gas engine E is in operation in the present embodiment, the main oxygen sensor 12 is in an active state, and the like. If the main feedback condition is not satisfied, that is, if step S1 is NO, the air-fuel ratio adjustment amount FAF is set to 0 in step S2, and the routine is ended.

When it is determined that the main feedback condition is satisfied (YES in step S1), the main feedback after step S2 is executed.

First, in step S2, the output V1 of the main oxygen sensor 12 is fetched. Then, it is determined whether or not the fetched output V1 is smaller than the comparison voltage VR1, in other words, the lean state or the rich state (step S4). If the output V1 is smaller than the comparison voltage VR1 (lean state: YES in step S4), it is determined in step S5 whether the flag F1 has a positive value. This means determining whether or not the air-fuel ratio was not reversed in the previous control routine.

When the flag F1 has a positive value (YES in step S5), the process proceeds to step S6, the value of the first delay counter CDLY1 is decremented by 1, and the step S6 is executed.
Proceed to 7. On the other hand, when the flag F1 has a negative value (NO in step S5), it is determined that the air-fuel ratio has been inverted, and the processes of steps S8 and S9 are performed. That is, in step S8, the valve opening degree F shown in FIG.
The constant KR1 that defines the rate of change of AF is the reverse flag RF1. Then, in step S9, the first
The value of the delay counter CDLY1 is set to a first lean delay time TDL1 described later. Then, the process proceeds to step S7.

In step S7, another constant KL1 which defines the changing speed of the air-fuel ratio correction amount FAF shown in FIG. 2D is set as the flag F1. Then, the process proceeds to step S10, and the first delay counter CDLY
It is determined whether the value of 1 has become zero, that is, whether the delay time has elapsed.

When step S4 is NO, that is, when the output V1 is larger than the comparison voltage VR1 and in the rich state, the routine proceeds to step S11, where it is determined whether or not the flag F1 has a negative value, that is, in the previous control routine. It is determined whether the air-fuel ratio has not been reversed. If the flag F1 has a negative value (YES in step S11)
S), the flow proceeds to step S12, and the first delay counter C
Decrease the value of DLY1 by 1. Then, the process proceeds to step S13.

On the other hand, if the flag F1 has a positive value (NO in step S11), it is determined that the air-fuel ratio has been reversed in the previous control routine, and steps S14, 15 are executed.
Process. That is, in step S14, the constant KL1 defining the rate of change of the valve opening degree FAF shown in (D) of FIG. 2 becomes the reverse flag RF1. Then, in step S15, the value of the first delay counter CDLY1 is set to the first rich delay time TD to be described later.
Set to R1. Then, it progresses to step S13.

In step S13, a constant KR1 which defines the changing speed of the air-fuel ratio correction amount FAF shown in FIG. 2D.
Is set as the flag F1. Then, in step S10, it is determined whether or not the value of the first delay counter CDLY1 becomes zero.

At step S10, if the delay time has just passed and the value of CDLY1 becomes zero (step S10).
Is YES), the air-fuel ratio adjustment amount FAF is skipped as shown in FIG. Therefore, in step S16, the air-fuel ratio adjustment amount FAF is set to a value obtained by adding the skip amount (F1 × RS1) to the immediately preceding numerical value, and this control routine ends.

If the value of CDLY1 is not zero in step S10, the process proceeds to step S17, and it is determined whether the value of CDLY1 is positive or negative, and thus it is determined whether the delay time (delay) is completed. When the value of CDLY1 is positive and before the delay is completed (YES in step S17), the air-fuel ratio correction amount FAF is set immediately before the amount of (reverse flag RF1) × {inclination k1} shown in (D) of FIG. The value added to the numerical value of FAF is set (step S18), and this control routine ends. On the other hand, if step S17 is NO (C
If the value of DLY1 is negative and the delay is completed), the process proceeds to step S19, and the amount of {(flag F1) × (inclination k1 shown in FIG. 2D)} is added to the value of the immediately preceding FAF. Set and end the control routine.

By the main feedback process described above, the air-fuel ratio adjustment amount FAF is set appropriately. As a result, in the three-way catalyst 11, the air-fuel ratio with which the emission is most effective can be obtained.

The first rich or lean delay time in the main feedback depends on the sub oxygen sensor 13 (FIG. 1).
Is set by sub-feedback described below with reference to FIGS. The control routine for the main feedback is executed, for example, every 4 ms, but the sub feedback does not necessarily have to be executed for each control routine for the main feedback, and is started or executed, for example, every 1 s.

The flow of this sub-feedback is shown in FIG. In FIG. 4, first, it is determined whether or not the condition for sub-feedback is satisfied (step S31). As the conditions for the sub feedback, all the conditions that the main feedback condition is satisfied (YES in step S1 of FIG. 3) and that the sub oxygen sensor 13 (FIG. 1) is in the active state are all included. Satisfaction, etc. can be mentioned.

If step S31 is NO, that is, if the condition for sub-feedback is not satisfied, or if sub-feedback is not executed, the control routine ends. On the other hand, when the condition for sub feedback is satisfied (YES in step S31), the process proceeds to step S32, and the sub oxygen sensor output V2 shown in FIG.

The output V2 taken in is step S33.
At the second comparison voltage VR2. In other words, the output V2 of the sub oxygen sensor 13 is set to the second comparison voltage V
By comparing with R2, it is determined whether the air-fuel ratio detected by the sub oxygen sensor 13 is lean or rich.

When the sub oxygen sensor output V2 is smaller than the second comparison voltage VR2, that is, the sub oxygen sensor 13
When the air-fuel ratio detected in step S3 is lean (YES in step S33), it is determined in step S34 whether the second flag F2 is greater than zero. This is to determine whether or not the air-fuel ratio has been reversed in the immediately preceding routine, and if not, step S3
4 becomes YES. In that case, proceed to step S35,
The value of the second delay counter CDLY2 is decremented by 1, and the process proceeds to step S36.

If NO at step S34, that is, if the air-fuel ratio has been reversed in the immediately preceding routine,
In step S37, the constant KR2 that defines the changing speed of the delay correction value DLTD shown in FIG. 5D is set to the second reverse flag RF2. Then, the second lean delay time TDL2 is set to the second delay counter CD
Set to the value of LY2 (step S38), step S3
Go to 6.

In step S36, (D) of FIG.
Another constant KR2 that defines the changing speed of the delay correction value DLTD is set as the second flag F2. Then, the process proceeds to step S39.

When the sub oxygen sensor output V2 is larger than the second comparison voltage VR2, the air-fuel ratio detected by the sub oxygen sensor 13 is in the rich state, and when step S33 becomes NO, the same as step S34. To step S40
At, it is determined whether the flag F2 is smaller than zero. If the second flag F2 is negative and it is estimated that the air-fuel ratio has not been inverted in the immediately preceding routine (YES in step S40), the second delay counter CDLY
Decrement the value of 2 by 1 (step S41), then step S
Proceed to 42.

If NO in step S40, that is, if the air-fuel ratio has been inverted in the immediately preceding routine,
In step S43, the constant KR2 is set to the second reverse flag RF2. Then, the second rich delay time TDR2 is set to the value of the second delay counter CDLY2 (step S44), and the process proceeds to step S42.

In step S42, the constant KR2 is set as the second flag F2, and then in step S3.
Proceed to 9.

In step S39, it is determined whether or not the second delay counter CDLY2 is zero.
When the delay time has just passed and the value of CDLY2 becomes zero (YES in step S10), the delay correction value DLT
D is skipped as shown in FIG. That is, in step S45, the delay correction value DLTD is
The value is set to the value obtained by adding the skip amount (F2 × RS2) to the previous value. Then, the process proceeds to step S46.

If the value of CDLY2 is not zero in step S39, the flow advances to step S47 to determine whether the value of CDLY2 is positive or negative, and thus it is determined whether or not the second delay time (delay) is completed. to decide. CDLY2 is negative and the delay has not been completed (step S47 returns YE
S), the delay correction value DLTD is {(FIG. 5 (D)
DLTD slope k2) × (reverse flag RF
2)} is added to the immediately preceding delay correction value DLTD and set (step S48). On the other hand, CDLY2
Is positive and the delay is completed (step S47).
Is NO), a value obtained by adding a numerical value {(inclination k2) × (flag F2)} to the immediately preceding delay correction value DLTD is set as a new delay correction value DLTD (step S4).
9). Then, after step S48 or step S49 is executed, the process proceeds to step S46.

When executing step S46, the delay correction value DLTD has already been set (step S45,
See S48 and S49). In step S46, it is determined whether the delay correction value DLTD is positive or negative. If the delay correction value DLTD is positive (YES in step S46).
S), the first rich delay time TDR1 becomes a value obtained by adding the delay correction value DLTD to the constant a, and the first lean delay time TDL1 is set to the constant b as it is (step S50). On the other hand, if the delay correction value DLTD is negative (NO in step S46), the constant a is set as it is as the first rich delay time TDR1 and the constant b is set as the first lean delay time TDL1 as shown in step S51. Is set to a value obtained by subtracting the delay correction value DLTD from.

In this way, the first rich delay time TDR1
At the stage when the first lean delay time TDL1 is set, the sub-feedback control routine is completed.

By executing the main feedback control routine and the sub-feedback control routine as described above, the air-fuel ratio control of the gas engine E shown in FIG. 1 is suitably performed.

Next, control for raising the threshold level of the main feedback, that is, the first comparison voltage VR1 in response to the deterioration of the three-way catalyst 11 will be described with reference to FIGS.

In FIG. 7, the comparison voltage VR1 when the three-way catalyst is not deteriorated is the main oxygen sensor characteristic curve s.
Is set as the output when the λ window at 1.0 is 1.0. On the other hand, when the three-way catalyst deteriorates and the λ window is richly deviated by Δλ to 0.995,
The output when the main oxygen sensor characteristic curve s is λ = 0.995 (reference numeral NVR1 in FIG. 7) is set as a new comparison voltage. As a result, the comparison voltage increases by the amount indicated by ΔVR in FIG.

FIG. 6 shows a flow chart of a control routine for increasing the comparison voltage by an amount indicated by ΔVR (FIG. 7). First, in step S101, it is determined whether feedback is established. Here, the conditions for the feedback to be satisfied are that the main feedback and the sub feedback are satisfied, and the gas engine E
Is working, etc.

If the feedback is established (YES in step S101), the output V2 of the sub oxygen sensor 13 is output.
Is taken in (step S102). Then, the vibration frequency H of the sub oxygen sensor output V2 and the amplitude A of the sub oxygen sensor output V2 whose lower limit of amplitude is near the threshold level (second comparison voltage) VR2 of the sub feedback are detected. Is performed (step S103).

Here, when the vibration frequency H of the sub oxygen sensor output V2 is 2 Hz or more, the detected output V2 has a high probability of noise, so it is unnecessary in the control routine shown in FIG. Further, when the amplitude A of the output of the sub oxygen sensor is 100 mV or less, the deterioration of the three-way catalyst has not progressed so much, so the threshold level of the main feedback, that is, the first comparison voltage VR1 is increased to cause the emission. There is no need to improve. Therefore, in step S104, the sub oxygen sensor output V
When the vibration frequency H of 2 is smaller than 2 Hz and the amplitude A is larger than 100 mV (YES in step S104), the control routine of step S105 and thereafter is executed, but otherwise, the control routine is ended ( Step S104 is NO). In other words, step S10
When 4 is YES, it means that the three-way catalyst is deteriorated to the extent that it is necessary to raise the first comparison voltage VR1 to improve the emission.

If step S104 is YES, it is determined whether or not the sub oxygen sensor is deteriorated (step S105). When judging the deterioration of the sub oxygen sensor,
For example, when the air-fuel ratio correction amount FAF is increased and then narrowed down, and when the air-fuel ratio correction amount FAF is increased, the sub oxygen sensor output V2 becomes larger than the upper threshold level (not shown), and the air-fuel ratio correction amount FAF is increased. If the sub-oxygen sensor output V2 becomes smaller than the lower threshold level (not shown) when squeezing is made small, the sub-oxygen sensor can be determined to be normal (not deteriorated).

If the sub oxygen sensor has not deteriorated (YES in step 105), it is determined whether the main oxygen sensor has deteriorated (step S106). When determining the deterioration of the main oxygen sensor, as in the case of the sub oxygen sensor, the air-fuel ratio correction amount FAF of the air-fuel ratio control valve 8 (FIG. 1) is increased and then narrowed down to increase the air-fuel ratio adjustment amount FAF. Sometimes the main oxygen sensor output V
1 becomes larger than the upper threshold level (not shown), and when the main oxygen sensor output V1 becomes smaller than the lower threshold level (not shown) when the air-fuel ratio adjustment amount FAF is narrowed down, A method of determining that the main oxygen sensor is normal (not deteriorated) can be adopted. In addition to this, by increasing the skip amount, the main oxygen sensor V
If the amplitude of 1 is increased, the upper limit of the amplitude is larger than the upper threshold level, and the lower limit of the amplitude is smaller than the lower threshold level, it is possible to determine that the main oxygen sensor is not deteriorated.

If the main or sub oxygen sensor has deteriorated (either of steps S105 and S106 is N).
O), the control routine ends. On the other hand, if the main and sub oxygen sensors have not deteriorated (step S105,
106 is YES), the operation is performed to increase the first comparison voltage VR1. In that case, first, in step S107, a value obtained by multiplying the first comparison voltage VR1 at that point in time by the amplitude k of the sub oxygen sensor output by a constant k is set, and this value is set as a new comparison voltage VR1. .. In this embodiment, for example, 1.0 is adopted as the constant k. However, this constant k can be arbitrarily selected on a case-by-case basis.

Next, the new comparison voltage VR1 is 0.70.
It is determined whether or not it is smaller than V (step S108),
If YES, the control routine is immediately ended, and if NO, a new comparison voltage VR1 is set to 0.70 V (step S109), and then the control routine is ended. As a result, the threshold level of the main feedback, that is, the first comparison voltage VR1 rises according to the degree of deterioration of the three-way catalyst.

It should be noted that various specific numerical values or critical values adopted above can be arbitrarily selected on a case-by-case basis.

[0062]

The effects of the present invention are listed below.

(1) When the ternary catalyst is deteriorated and the λ window is deviated to the rich side, the threshold level of the main feedback is increased to the potential corresponding to the λ window deviated to the rich side, thereby controlling the control. The amplitude is reduced and the period is shortened, and the emission is improved.

(2) It is possible to improve the emission without replacing the three-way catalyst.

(3) The life of an expensive three-way catalyst can be extended, and as a result, the cost required for replacing the three-way catalyst can be saved and the running cost can be kept low.

[Brief description of drawings]

FIG. 1 is a diagram showing an overall configuration of a gas engine in which the present invention is implemented.

2 is a characteristic diagram showing a temporal relationship between an output of a main oxygen sensor, a rich / lean determination, a delay process, and a valve opening degree in the main feedback of the gas engine of FIG.

FIG. 3 is a diagram showing a flowchart of main feedback.

FIG. 4 is a diagram showing a flowchart of sub-feedback.

FIG. 5 is a characteristic diagram showing a temporal relationship between the output of the main oxygen sensor, the rich / lean determination, the delay process, and the valve opening degree in the sub feedback.

FIG. 6 is a diagram showing a flowchart of control for raising a threshold level in main feedback.

FIG. 7 is a characteristic diagram showing an output characteristic of a main oxygen sensor with respect to a λ window.

[Explanation of symbols]

1 ... Intake passage 2 ... Fuel passage 3 ... Air intake 4 ... Mixer 5 ... Throttle valve 6 ... Pressure sensor 7 ... Bypass passage 8 ... Air-fuel ratio control valve 9 ... Revolution sensor 10 ... Exhaust passage 11 ... Three-way catalyst 12 ... Main oxygen sensor 13 ... Sub oxygen sensor 14 ... Temperature sensor 15 ... Control device 16 ... -Total gas consumption amount calculation means 17 ... Bypass gas amount calculation means 18 ... Bypass ratio table 19 ... Catalyst deterioration determination means 20 ... Control constant determination means 21 ... Air-fuel ratio correction amount calculation means 22・ ・ ・ Bypass gas amount calculating means 23 ・ ・ ・ Air-fuel ratio adjusting means V1 ・ ・ ・ Main oxygen sensor output V2 ・ ・ ・ Sub oxygen sensor output TDL, TDL1, TDL2, TDR, TDR1, TD
R2 ... Delay time FAF ... Air-fuel ratio correction amount KR1, KL1 ... Constant that regulates change rate of air-fuel ratio control valve opening DLTD ... Delay correction value KR2, KL2 ... Delay correction value Another constant that regulates the rate of change VR1 ... First comparison voltage (threshold level in main feedback) VR2 ... Second comparison voltage k ... Increase threshold level in main feedback A constant that multiplies the amplitude of the sub oxygen sensor output when

Claims (1)

[Claims] 1. A three-way catalyst arranged in an exhaust passage for purifying exhaust gas, a first oxygen sensor arranged upstream of the three-way catalyst, and arranged downstream of the three-way catalyst. The second done
In response to the outputs of the first oxygen sensor and the second oxygen sensor, whether the air-fuel ratio of the gas engine is rich or lean to the stoichiometric air-fuel ratio based on the oxygen concentration in the exhaust gas. The air-fuel ratio control device for controlling the air-fuel ratio of the gas engine based on the result, and in the air-fuel ratio control method of the gas engine including, in the main feedback corresponding to the degree of deterioration of the three-way catalyst. A method of controlling an air-fuel ratio of a gas engine, comprising: a step of increasing a threshold level.