Air-fuel ratio control apparatus for engine
This application is a division of application Ser. No. 690,825, filed Apr. 26, 1991, now U.S. Pat. No. 5,154,053. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a gas engine, and particularly relates to an air-fuel ratio control apparatus for a gas engine which operates to control an air-fuel ratio at a desired value by supplying therethrough at least one of intake air and a fuel gas downstream of a mixer, with the mixer being bypassed. 
     2. Description of the Related Art 
     As for gasoline engines, there is conventionally known an apparatus in which an air-fuel ratio is controlled to the vicinity of a theoretical air-fuel ratio (or a catalyzer window) in accordance with the output signal of an O 2  sensor disposed in the upper stream of a catalyzer, thereby improving the rate of purification by the catalyzer. 
     Further, there is an air-fuel ratio control apparatus for gasoline engine in which a change in characteristic of the output signal of an O 2  sensor provided in the upper stream of a catalyzer, or the like, is corrected in accordance with the output signal of an O 2  sensor provided in the lower stream of the catalyzer (for example, see JP-A-61-286550). 
     On the other hand, the present inventors have conducted experiments on engines using various gases such as a city gas and have revealed that an exhaust gas is not sufficiently mixed even in the lower stream of a catalyzer. This is caused by the fact that the gas is harder to mix in the air, as compared with the gasoline. The above phenomenon remarkably appears, especially, in an apparatus in which an air-fuel ratio is controlled by adjusting an intake air or a fuel gas which is supplied to the upper stream of a throttle valve, by-passing a mixer for mixing the intake air and the fuel gas. 
     Accordingly, in the case where such air-fuel ratio control as mentioned above is applied to a gas engine, there is a problem that the output of an O 2  sensor disposed in the upper or lower stream of a catalyzer changes depending upon the attachment position of the O 2  sensor, for example, variations of the attachment position thereof in a direction of circumference of an exhaust pipe, thereby giving rise to variations of the control performance. 
     SUMMARY OF THE INVENTION 
     The present invention aims at solving the above-described problem, and the object of the present invention is to provide an air-fuel control apparatus for a gas engine which is able to control an air-fuel ratio at a desired value with high precision, even when the bypass supply ratio varies as mentioned above. 
     The general arrangement of an air-fuel ratio control apparatus for a gas engine of the present invention has an embodiment shown in FIG. 2. 
     FIG. 2 shows an air-fuel ratio control apparatus for a gas engine. The invention includes: 
     a mixture for mixing intake air and a fuel gas for supply to a gas engine; 
     subsidiary supply means for supplying at least one of the intake air and the fuel gas downstream of the mixer, with the mixer being bypassed; 
     oxygen concentration sensors disposed in an exhaust system of the gas engine for detecting a concentration of oxygen in an exhaust gas exhausted from the gas engine; 
     operating condition detecting means for detecting operating conditions of the gas engine; 
     basic amount setting means for setting a basic amount of at least one of the intake air and the fuel gas supplied through the subsidiary supply means in accordance with the operating conditions of the gas engine; 
     correction amount setting means for setting a correction amount proportional to a total fuel gas supply rate including the fuel gas supplied directly to the mixer and the fuel gas supplied through the subsidiary supply means, in accordance with output signals of the operating condition detecting means and output signals of the oxygen concentration sensors; and 
     control amount setting means for setting a control amount of at least one of the intake air and the fuel gas supplied through the subsidiary supply means by making an addition of the correction amount to the basic amount. 
     The air-fuel ratio control apparatus for a an engine of the present invention further comprises a catalyzer disposed in the exhaust system of the gas engine for purifying the exhaust gas exhausted from the gas engine, and the oxygen concentration sensors include a first oxygen concentration sensor and a second oxygen concentration sensor disposed upstream and downstream, respectively, of the catalyzer. 
     With the above-described arrangement of the apparatus of the present invention, the mixer operates to produce a mixture gas containing intake air and a fuel gas. In addition, at least one of the intake air and the fuel gas is supplied through the subsidiary supply means to perform air-fuel ratio control so that the air-fuel ratio of a mixture gas supplied to the gas engine may have a desired value. 
     In order to perform the aforesaid air-fuel ratio control, a control amount of at least one of the intake air and the fuel gas supplied through the subsidiary supply means is calculated and set by the control amount setting means by making an addition of a correction amount produced by the correction amount setting means to a basic amount produced by the basic amount setting means, where the basic amount setting means calculates and sets the basic amount in accordance with the operating conditions of the gas engine, and the correction amount setting means calculates and sets the correction amount proportional to a total fuel gas supply rate including the fuel gas supplied directly to the mixer and the fuel gas supplied through the subsidiary supply means, in accordance with output signals of the operating condition detecting means and output signals of the oxygen concentration sensor. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the construction of the claimed invention. 
     FIG. 2 is a block diagram of an embodiment to which the present invention is applied; 
     FIG. 3 is a perspective view of a blade plate 14 or 15; 
     FIG. 4 is a cross section of the blade plate 14 or 15; 
     FIGS. 5 to 7 are flow charts useful in explaining the operation of the above embodiment; 
     FIG. 8 shows, in (a) to (i), time charts useful in explaining the operation of the above embodiment; 
     FIG. 9 is a graph showing a relation between an intake pressure PM and a total gas flow rate; 
     FIG. 10 is a graph showing a relation between an engine speed NE and an engine speed correction factor KNE; 
     FIG. 11 is a graph showing a relation between a duty ratio and a by-pass flow rate; 
     FIG. 12 is a graph showing a relation between a second lean integration constant and a deviation DLOXS; 
     FIG. 13 is a graph showing a relation between the intake pressure PM and a second comparison voltage; 
     FIG. 14 is a graph showing a relation between the intake pressure PM and the second lean integration constant; and 
     FIG. 15 is a diagram showing the distribution of an exhaust gas in a four-cylinder gas engine. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment, to which the present invention is applied, will now be explained on the basis of the drawings. 
     FIG. 2 is a block diagram of the present embodiment. Reference numeral 1 denotes a gas engine which uses a city gas as a fuel. An inlet system of the gas engine 1 is composed of an air cleaner 2 for cleaning an intake air and an inlet pipe 3 for introducing to the gas engine 1 a mixture gas of the intake air cleaned by the air cleaner 2 and a fuel gas supplied from a fuel gas supply source which is not shown. Further, the inlet pipe 3 is provided with a mixer 4 for mixing the intake air and the fuel gas to form a mixture gas which is slightly lean as compared with a theoretical air-fuel ratio and a throttle valve 5 for adjusting the amount of mixture gas to be supplied to the gas engine 1 (or a total gas flow rate). Also, there are provided a main supply path 6 which supplies the fuel gas from the gas supply source directly to the mixer 4 and a subsidiary supply path 7 which supplies the fuel gas from the gas supply source to a lower stream of the mixer 4. Further, the subsidiary supply path 7 is provided with a control valve 8 for air-fuel ratio control which adjusts the amount of fuel gas supplied from the subsidiary supply path 7 (or a by-pass flow rate) so that the air-fuel ratio of the mixture gas supplied to the gas engine 1 is controlled to a desired value. Also, there is provided a pressure sensor 9 which detects an intake pressure PM in a lower stream of the throttle valve 5. 
     On the other hand, an exhaust system of the gas engine 1 includes an exhaust pipe 10 for guiding an exhaust gas from the gas engine 1. A ternary catalyzer 11 for purifying harmful components contained in the exhaust gas is disposed in the exhaust pipe 10. Further, in upper and lower streams of the ternary catalyzer 11 are respectively provided first and second oxygen concentration sensors (O 2  sensors) 12 and 13 which detect the concentration of oxygen in the exhaust gas in order to detect the air-fuel ratio of the mixture gas supplied to the gas engine 1. Further, first and second blade plates 14 and 15 as mixing members for mixing the exhaust gas are respectively disposed in an upper stream of the first O 2  sensor 12 and between the ternary catalyzer 11 and the second O 2  sensor 13. The material of the blade plates 14 and 15 is, stainless steel (SUS304) in the present embodiment. FIG. 3 is a perspective view of the blade plate 14 or 15 and FIG. 4 is a cross section of the blade plate 14 or 15. In FIG. 3, A denotes a blade which corresponds to the radius of the inlet pipe and B denotes a mounting portion for attaching the blade plate 14 or 15 to the inlet pipe. Also, the blade plate 14 or 15 is provided with blades A which extend toward the upper and lower stream sides of the exhaust pipe 10, as shown in FIGS. 3 and 4. The blade A has a curved surface structure by which a scroll is caused to generate in the flow of the exhaust gas so that the exhaust gas is mixed. 
     Reference numeral 16 denotes a spark plug provided at a cylinder of the gas engine 1 and numeral 17 denotes a speed sensor for detecting the speed or number of rotation NE of the gas engine. 
     Reference numeral 20 denotes an electronic control unit (ECU) which sets the controlled variables for various actuators such as the above-mentioned control valve 8, spark plug 16, etc. and outputs control signals corresponding to the controlled variables. As well known, the ECU 20 is composed of a central processing unit (CPU) 20a which performs various operations, a read only memory (ROM) 20b in which a control program and so on are stored, a writable/readable random access memory (RAM) 20c which temporarily stores operation data and so on, an analog-digital converter (ADC) 20d which converts an analog signal into a digital signal, an input port 20e for taking sensor signals from the above-mentioned various sensors into the ECU 20, an output port 20f for outputting the control signals to the above-mentioned various actuators, and a bus 20g which interconnects these components. 
     In the following, a method of controlling the controlled variable for the control valve 8, that is, a method of controlling an air-fuel ratio of the gas engine will be explained by use of flow charts shown in FIGS. 5 to 7. FIG. 8 shows, in (a) to (i), a time chart of the present embodiment. 
     FIG. 5 is a flow chart showing a controlled variable calculation routine in which the controlled variable D for the control valve 8 is calculated. 
     Firstly or in step 301, a basic control amount DB is calculated by the following equation in accordance with an intake pressure PM detected by the pressure sensor 9 and the engine speed NE detected by the speed sensor 17: 
     
         DB←(PM-PMOS)×KPMB×KNE×KDB+DOS 
    
     where PMOS is a constant value corresponding to an offset of such a relation between the intake pressure PM and a total gas flow rate as shown in FIG. 9, KPMB is a conversion coefficient for converting the intake pressure PM into a duty ratio, KNE is an engine speed correction factor corresponding to the engine speed NE which satisfies such a relation with the engine speed correction factor KNE as shown in FIG. 10, KDB is a correction factor set in accordance with the intake pressure PM and the engine speed NE, and DOS is a constant value corresponding to an offset of such a relation between the duty ratio and a by-pass flow rate as shown in FIG. 11. 
     In subsequent step 302, a corrected controlled variable DF is calculated by the following equation in accordance with the intake pressure PM, the engine speed. NE and an air-fuel ratio correction factor which will be mentioned in later: 
     
         DF←(PM-PMOS)×KPMF×KNE×FAF 
    
     where KPMF is a value which is set by the following equation on the basis of the gradient α of the intake pressure PM versus total gas flow rate characteristic shown in FIG. 9 and the gradient β of the duty ratio versus by-pass flow rate shown in FIG. 11: 
     
         KPMF←α/β 
    
     Thus, a value obtained by the calculation in accordance with the equation (PM-PMOS)×KPMF×KNE is proportional to a total fuel gas supply rate, and hence it results that the correction amount has a value which is proportional to a total fuel gas supply rate and which is determined uniquely in accordance with the operating conditions of the gas engine 1. 
     In step 303, the controlled variable D is calculated in accordance with the thus calculated basic controlled variable DB and corrected controlled variable DF: 
     
         D←DB+DF. 
    
     In step 304, a control signal corresponding to the controlled variable is outputted to the control valve 8. 
     In this manner, the controlled variable calculation routine is completed. 
     Next, a method of setting the air-fuel ratio correction factor FAF will be explained. FIG. 6 is a flow chart showing a main air-fuel ratio feedback control routine in which the air-fuel ratio correction factor FAF is calculated on the basis of an output value V1 of the first O 2  sensor 12 (or a first output value) as shown in (a) of FIG. 8. This main air-fuel ratio feedback control routine is actuated at every predetermined time (for example, every 4 ms in the present embodiment). 
     Firstly or in step 401, the judgement is made of whether or not a main air-fuel feedback condition is satisfied. The main air-fuel ratio feedback condition is, for example, in the present embodiment, that the engine has been started up and the first O 2  sensor 12 is in an active state. In the case where the result of judgement in step 401 is that the main air-fuel ratio feedback condition is not satisfied, the flow proceeds to step 402 in which an air-fuel ratio correction factor FAF is set to 0 (FAF←0) . 
     On the other hand, in the case where the result of judgement in step 401 is that the main air-fuel ratio feedback condition is satisfied, a main air-fuel ratio feedback processing in and after step 403 is performed. Firstly or in step 403, a first output value V1 is taken in. In step 404, the judgement is made of whether or not the first output value V1 is not larger than a first comparison voltage VR1 (for example, 0.45 V in the present embodiment), that is, whether the air-fuel ratio is rich or lean. Namely, the first output value V1 as shown in (a) of FIG. 8 is judged as shown in (b) of FIG. 8. In the case where the first output value V1 is not larger than the first comparison valve VR1, that is, the air-fuel ratio is lean, the flow proceeds to step 405 in which the value of a first delay counter CDLY1 is decremented (CDLY1←CDLY1-1). 
     In subsequent steps 406 and 407, the first delay counter CDLY1 is subjected to a guard processing with a first lower limit TDR1. In particular, in step 406, the judgement is made of whether or not the first delay counter CDLY1 is smaller than the first lower limit TDR1. When the first delay counter CDLY1 is smaller than the first lower limit TDR1, the flow proceeds to step 407 in which the first delay counter CDLY1 is set to the first lower limit TDR1 again. 
     On the other hand, in the case where the result of judgement in step 404 is that the first output value V1 is larger than the first comparison voltage VR1, that is, the air-fuel ratio is rich, the flow proceeds to step 408 in which the value of the first delay counter CDLY1 is incremented (CDLY1←CDLY1+1). In subsequent steps 409 and 410, the first delay counter CDLY1 is subjected to a guard processing with a first upper limit TDL1. More specifically, in step 409, the judgement is made of whether or not the fist delay counter CDLY1 is larger than the first upper limit TDL1. When the first delay counter CDLY1 is larger than the first upper limit TDL1, the flow proceeds to step 410 in which the first delay counter CDLY1 is set to the first upper limit TDL1 again. 
     The above-mentioned first lower limit TDR1 is a first rich delay time for holding the judgement of the output of the first O 2  sensor 12 as being in a lean state notwithstanding the occurrence of a change from the lean state to a rich state, as shown in (c) of FIG. 8. The first lower limit TDR1 is defined by a negative value. Also, the first upper limit TDL1 is a first lean delay time for holding the judgement of the output of the first O 2  sensor 12 as being in a rich state notwithstanding the occurrence of a chance from the rich state to a lean state, as shown in (c) of FIG. 8. The first upper limit TDL1 is defined by a positive value. Zero is taken as a reference level of the first delay counter CDLY1, and an air-fuel ratio after a delay processing is regarded as being rich when the first delay counter CDLY1 is positive and is regarded as being lean when the first delay counter CDLY1 is negative. In step 411, the judgement is made of whether or not the sign of the first delay counter CDLY1 set as mentioned above is inverted, that is, the air-fuel ratio after the delay processing is inverted. In the case where the air-fuel ratio after the delay processing is inverted, a skip processing in steps 412 to 414 is performed. Firstly or in step 412, the judgement is made of whether or not the inversion is one from a rich state to a lean state. In the case where the judgement as being the inversion one from a rich state to a lean state is made in step 412, the flow proceeds to step 413 in which the air-fuel ratio correction factor FAF is increased by a first amount of skip RS1 (FAF←FAF+RS1). Also, in the case where the judgement as being the inversion one from a lean state to a rich state is made in step 412, the flow proceeds to step 414 in which the air-fuel ratio correction factor FAF is decreased by the first amount of skip RS1 (FAF←FAF-RS1). 
     On the other hand, in the case where the result of judgement in step 411 is that the air-fuel ratio after the delay processing is not inverted, an integration processing in steps 415 to 417 is performed. Firstly or in step 415, the judgement is made of whether or not the first delay counter CDLY1 is not larger than 0, that is, whether the air-fuel ratio is in a rich state or a lean state. In the case where the judgement as being a lean state is made in step 415, the flow proceeds to step 416 in which the air-fuel ratio correction factor is increased by a first integration constant K1 (FAF←FAF+K1). Also, in the case where the judgement as being a rich state is made in step 415, the flow proceeds to step 417 in which the air-fuel ratio correction factor FAF is decreased by the first integration constant K1 (FAF←FAF-K1). 
     The first integration constant K1 is set to be sufficiently small as compared with the first amount of skip RS1. Accordingly, in the case where the air-fuel ratio is in a lean state, the fuel gas supplied is gradually increased since the air-fuel ratio correction factor FAF is gradually increased, as shown in (d) of FIG. 8. Also, in the case where the air-fuel ratio is in a rich state, the fuel gas supplied is gradually decreased since the air-fuel ratio correction factor FAF is gradually decreased. 
     In this manner, the main air-fuel ratio feedback control routine is completed. 
     FIG. 7 is a flow chart showing a subsidiary air-fuel ratio feedback control routine in which the first delay times TDR1 and TDL1 as the amounts of correction for output signal are calculated on the basis of an output value V2 of the second O 2  sensor 13 (or a second output value) shown in (e) of FIG. 8. This subsidiary air-fuel ratio feedback control routine is activated at every predetermined time (for example, 1 s in the present embodiment). 
     Firstly or in step 501, the judgement is made of whether or not a subsidiary air-fuel ratio feedback condition is satisfied, that is, whether or not a subsidiary air-fuel ratio feedback control should be made. The case where the subsidiary air-fuel ratio feedback control condition is satisfied, corresponds to, for example, the case where there are satisfied all of conditions 
     1 that the main air-fuel ratio feedback condition is satisfied, 
     2 that the second O 2  sensor 13 is in an active state, and 
     3 that the ternary catalyzer 11 is deteriorated. 
     In the case where the result of judgement in step 501 is that the subsidiary air-fuel ratio feedback condition is not satisfied, the subsidiary air-fuel ratio feedback control in and after step 504 is not performed and the flow proceeds to step 502 in which a learning value DLTDAV, which will be mentioned later, is substituted for the preceding delay correction value DLTDO to prepare for the next subsidiary air-fuel ratio feedback control (DLTDO←DLTDAV). In subsequent step 503, the leaning value DLTDAV is substituted for a delay correction value DLTD (DLTD←DLTDAV), and the flow thereafter proceeds to step 523. 
     On the other hand, in the case where the result of jdugement in step 501 is that the subsidiary air-fuel ratio feedback condition is satisfied, that is, the subsidiary air-fuel ratio feedback control should be made, a processing in and after step 504 is performed. 
     Firstly or in step 5041 the second output value V2 is taken in. In step 505, a deviation DLOXS (←V2-VR2) between the second output value V2 and a second comparison voltage VR2 is calculated. In subsequent step 506, the judgement is made whether or not the deviation DLOXS is smaller than 0, that is, whether the air-fuel ratio is rich or lean, as shown in (f) of FIG. 8. In the case where the deviation DLOXS is smaller than 0, that is, the air-fuel ratio is lean, the flow proceeds to step 507 in which the value of a second delay counter CDLY2 is decremented (CDLY2←CDLY2-1). In subsequent steps 508 and 509, the second delay counter is subjected to a guard processing with a second lower limit TDR2, and the flow thereafter proceeds to step 513. More especially, in step 508, the judgement is made of whether or not the second delay counter CDLY2 is smaller than the second lower limit TDR2. When the second delay counter CDLY2 is smaller than the second lower limit TDR2, the flow proceeds to step 509 in which the second delay counter CDLY2 is set to the second lower limit TDR2 again. 
     On the other hand, in the case where the result of judgement in step 506 is that the deviation DLOXS is equal to or larger than 0 that is, the air-fuel ratio is rich, the flow proceeds to step 510 in which the value of the second delay counter CDLY2 is incremented (CDLY2←CDLY2+1). In subsequent steps 511 and 512, the second delay counter CDLY2 is subjected to a guard processing with a second upper limit TDL2, and the flow thereafter proceeds to step 513. More especially, in step 511, the judgement is made of whether or not the second delay counter CDLY2 is larger than the second upper limit TDL2. When the second delay counter CDLY2 is larger than the second upper limit TDL2, the flow proceeds to step 412 in which the second delay counter CDLY2 is set to the second upper limit TDL2. 
     The above-mentioned second lower limit TDR2 is a second rich delay time for holding the judgement of the output of the second O 2  sensor 13 as being in a lean state notwithstanding the occurrence of a change from the lean state to a rich state, as shown in (g) of FIG. 8. The second lower limit TDR2 is defined by a negative value. Also the second upper limit TDL2 is a second lean delay time for holding the judgement of the output of the second O 2  sensor 13 as being in a rich state notwithstanding the occurrence of a change from the rich sate to a lean state. The second upper limit TDR2 is defined by a positive value. Zero is taken as a reference level of the second delay counter CDLY2, and an air-fuel ratio after a delay processing is regarded as being rich when the second delay counter CDLY2 is positive and is regarded as being lean when the second delay counter CDLY2 is negative. 
     In step 513, the judgement is made of whether or not the second delay counter CDLY2 is inverted, that is, whether or not the air-fuel ratio after the delay processing is changed. In the case where the air-fuel ratio after the delay processing is changed, the flow proceeds to step 514 in which the mean of the preceding delay correction value DLTDO and a delay correction value DLTD is substituted for a learning value DLTDAV (DLTDAV←(DLTDO+DLTD)/2). In subsequent step 515, the delay correction value DLTD is substituted for the preceding delay correction value DLTDO (DLTDO←DLTD), and the flow thereafter proceeds to step 516. In step 516, the judgement is made of whether or not the inversion is one from a rich state to a lean state. In the case where the judgement as being the inversion from a rich state to a lean state is made in step 516, the flow proceeds to step 517 in which the delay correction value DLTD is decreased by a second amount of rich skip SSR (DLTD←DLTD-SSR), and the flow thereafter proceeds to step 523. Also, in the case where the judgement as being the inversion from a lean state to a rich state is made in step 516, the flow proceeds to step 518 in which the delay correction value DLTD is increased by the second amount of lean skip SSL (DLTD←DLTD+SSL), and the flow thereafter proceeds to step 523. The second amount of rich skip SSR is set to a value not smaller than the second amount of lean skip SSL. (In the present embodiment, the second amount of rich skip SSR and the second amount of lean skip SSL are set to the same value.) 
     On the other hand, in the case where the result of judgement in step 513 is that the air-fuel ratio after the delay processing is not inverted, the flow proceeds to step 519 in which the judgement is made of whether or not the second delay counter CDLY2 is not larger than 0, that is, whether the air-fuel ratio is in a rich state or a lean state. In the case where the judgement as being in a lean state is made in step 519, the flow proceeds to step 520 in which the delay correction value DLTD is decreased by a second rich integration constant SKR (DLTD←DLTD-SKR), and the flow thereafter proceeds to step 523. In the present embodiment, the second rich integration constant SKR is a predetermined value. Also, in the case where the judgement as being in a rich state is made in step 519, the flows proceeds to step 521 in which a lean integration constant SKL is set in accordance with the deviation DLOXS. FIG. 12 is a graph showing a relation between the deviation DLOXS and the lean integration constant SKL. In subsequent step 522, the delay correction value DLTD is increased by the second lean integration constant set in step 521 (DLTD←DLTD+SKL), and the flow thereafter proceeds to step 523. 
     In step 523, the detection is made of whether the delay correction value DLTD set as mentioned above is smaller than a reference value DLTD1. In the case where the delay correction value DLTD is smaller than the reference value DLTD1, the flow proceeds to step 524 in which a first lean delay time TDL1 is set to the minimum lean value TDLMIN. In subsequent step 525, the value of addition of the delay correction value DLTD and an initial rich value TDRO is substituted for the first rich delay time TDR1 (TDSR1←TDRO+DLTD), and a guard processing in steps 526 and 527 is thereafter performed. More especially, in step 526, the judgement is made of whether or not the first rich delay time TDR1 is smaller than the lower limit TR1. In the case where the first rich delay time TDR1 is smaller than the lower limit TR1, the flow proceeds to step 527 in which the first rich delay time TDR1 is set to the lower limit TR1 again (TDR1←TR1), and the present routine is completed. 
     On the other hand, in the case where the delay correction value DLTD is equal to or larger than the reference value DLTD1, the flow proceeds to step 528 in which the first lean delay time TDL1 is set by the following equation: 
     
         TDL1←TDLO+(DLTD-100) 
    
     where TDL0 is an initial lean value. In subsequent step 529, the first rich delay time TDR1 is set to the minimum rich value TDRMIN, and a guard processing in steps 530 and 531 is performed. More especially, in step 530, the judgement is made of whether or not the first lean delay time TDL1 is larger than the upper limit TL1. In the case where the first lean delay time TDL1 is larger than the upper limit TL1, the flow proceeds to step 531 in which the first lean delay time TDL1 is set to the upper limit TLl again (TDL1←TL1), and the present routine is completed. 
     The second integration constants SKR and SKL are set to be sufficiently small as compared with the second amounts of skip SSR and SSL. Therefore, in the case where the air-fuel ratio is in a lean state, the first rich delay time TDR1 is gradually increased or the first lean delay time TDL1 is decreased since the delay correction amount DLTD is gradually increased, as shown in (h) of FIG. 8. Also, in the case where the air-fuel ratio is in a rich state, the first rich delay time TDR1 is gradually decreased or the first lean delay time TDL1 is increased since the delay correction amount DLTD is gradually decreased. Accordingly, the center of control of the air-fuel ratio of a mixture gas supplied to the gas engine 1 is controlled so that it takes a theoretical air-fuel ratio λ0, as shown in (i) of FIG. 8. 
     Further, an exhaust gas exhausted from the gas engine 1, for example, in the case of four cylinders, has an air-fuel ratio distribution for each cylinder in regions A1 to A4 in a circumferential direction with respect to a cross section of the exhaust pipe 10, as shown in FIG. 15. As for the air-fuel ratio distributions produced for the respective regions A1 to A4, a scroll is generated in the flow of the exhaust gas by the four blades A of the blade plates 14 and 15, which are the same in number as the number of cylinders, so that the exhaust gas is sufficiently mixed. Especially, by providing the blade plate 15 between the catalyzer 11 and the second O 2  sensor 13 disposed in the lower stream thereof, it is possible to eliminate an unevenness that the flow velocity of the exhaust gas passing through the catalyzer 11 is fast in a central portion and slow in the vicinity of a wall surface of the exhaust pipe 10. Accordingly, in an upper stream of the second O 2  sensor 13, the air-fuel ratio becomes an average value for all of the cylinders. Therefore, even if the attachment position of the second O 2  sensor 13 is changed, no variations are produced in the output of the O 2  sensor, thereby making it possible to eliminate variations of the control performance. 
     The foregoing embodiment has a structure in which the subsidiary supply path 7 is opened in an upper stream of the throttle valve 5 so that the fuel gas is by-passed to the upper stream of the throttle valve 5. However, there may be employed a structure in which the fuel gas is by-passed to a lower stream of the throttle valve 5 or a structure in which the intake air is by-passed in lieu of the fuel gas. 
     Also, in the foregoing embodiments the second comparison voltage VR2 is a predetermined value. However, the second comparison voltage VR2 may be set in accordance with the intake pressure PM by use of a characteristic as shown in FIG. 13. 
     Further, in the foregoing embodiment, the second lean integration constant SKL is set in accordance with the deviation DLOXS. However, the second lean integration constant SKL may be set in accordance with the intake pressure PM by use of a characteristic as shown in FIG. 14. 
     In the foregoing embodiment, the attachment positions of the first and second blade plates 14 and 15 are in an upper stream of the first O 2  sensor 12 and between the ternary catalyzer 11 and the second O 2  sensor 13. However, the attachment position of the first blade plate 14 may be between the first O 2  sensor 12 and the ternary catalyzer 11. Also, a blade plate may be provided only in the upper stream of the first O 2  sensor 12. Further, in the case of a system having only one O 2  sensor, a blade plate may be provided on an upper stream side of that O 2  sensor. 
     As explained in detail in the foregoing description, in the air-fuel ratio control apparatus for a gas engine according to the present invention, the control amount setting means operates to calculate and set a control amount of at least one of intake air and a fuel gas supplied through the subsidiary supply means by making an addition of a correction amount worked out by the correction amount setting means to a basic amount worked out by the basic amount setting means, where the basic amount setting means operates to calculate and set the basic amount in accordance with the operating conditions of the gas engine, and the correction amount setting means operates to calculate and set the correction amount proportional to a total fuel gas supply rate including the fuel gas supplied directly to the mixer and the fuel gas supplied through the subsidiary supply means, in accordance with output signals of the operating condition detecting means and output signals of the oxygen concentration sensors. 
     Thus, the present invention has an excellent advantage in that the air-fuel ratio of a gas engine can be controlled at a desired value with high precision, even when the bypass supply ratio varies. 
     In addition, the device of the present invention further comprises the downstream oxygen concentration sensor disposed down stream of the catalyzer in addition to the upstream oxygen concentration sensor disposed upstream of the catalyzer, where both oxygen concentration sensors are provided to detect a concentration of oxygen in an exhaust gas to thereby decide the level of an air-fuel ratio of a mixture gas supplied to the as engine.