Patent Publication Number: US-6910460-B2

Title: Engine air-fuel ration control method with venturi type fuel supply device and fuel control appliance including the method

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an engine air-fuel ratio control method with a venturi type fuel supply device and a fuel control appliance including the method. 
   2. Prior Art 
   An air-fuel ratio control method with a venturi type fuel supply device and a fuel control appliance including the method are well known art. For example, the Japanese Application Patent Laid-open Publication No. 2000-18100 discloses a gas fuel engine with a venturi type fuel supply device comprising a venturi chamber located in the upstream of a throttle valve and a passage for supplying fuel into the venturi chamber, wherein CNG (compressed natural gas) is used as the gas fuel. This fuel supply device comprises a 3-port solenoid valve provided on the venturi chamber side in the passage for supplying fuel, a bypass passage connecting the 3-port solenoid valve and an intake system in the downstream of the throttle valve of the engine, and a control means for switching the 3-port solenoid valve at the time of starting of the engine so as to let the gas fuel into the bypass passage. Thereby it aims to improve the startability of the engine, in particular, the startability under low temperature. 
   Furthermore, the fuel supply device is also provided with a sub-injector in the intake system in the downstream of the throttle valve of the engine, and at the time of acceleration of the engine, the sub-injector is turned on so as to correct the supply quantity of the gas fuel, thereby keeps the operating condition of the engine favorable. 
   SUMMARY OF THE INVENTION 
   (Problems to be Solved by the Invention) 
   As explained above, a venturi type fuel supply device according to the prior art aims to improve the operating condition of an engine in start-up or acceleration by paying attention only to the flow rate of gas fuel at the time of start-up or acceleration. On a real vehicle equipped with the engine, however, the external load to the engine changes as the electrical switches or the like of the air-conditioner and lights of the vehicle are turned ON/OFF irrespective of whether the car is on idling or not. For example, if the air-conditioner switch is turned ON and consequently the external load is applied, required idling air flow rate (mixture air flow rate) becomes higher to keep the engine speed corresponding to the load. In the above-mentioned gas fuel engine with a venturi type fuel supply device, however, this issue is not considered and so an ignition failure may likely be caused. 
   By providing a bypass passage bypassing the throttle valve and also a bypass valve (ISC valve: Idle speed control valve) for controlling the flow area of the bypass passage and by adjusting the bypass valve opening, using a suitable control means, in accordance with a change in the external load, the required idling air flow rate (mixture air flow rate) can be adjusted higher or lower. However, in the case of opening the ISC valve and increasing the air quantity, the venturi chamber pressure decreases as the idling air flow rate increases because the pressure is drawn out by the downstream intake pipe pressure. If the venturi chamber pressure decreases, the gas fuel flow incoming from the fuel passage increases, thereby the air-fuel ratio becomes rich, and a “rich” ignition failure depends on the excessiveness of the ratio. Furthermore, exhaust gas results in deteriorated emission. These problems may arise not only on idling but also on non-idling. 
   An object of the present invention is to provide an air-fuel ratio control method of an engine with a venturi type fuel supply device and a fuel control appliance including its method, even if the external load changed, that are capable of supplying air-fuel mixture for keeping suitable engine speed corresponding to the load without changing the air-fuel ratio to a large degree, thereby minimizes a change of the engine speed and prevents ignition failure, further restrains deteriorated emission of exhaust gas. 
   Another object of the present invention is to restrain a driver&#39;s torque variation feeling by setting a transition processing for controlling the variation of the air-fuel ratio. Further another object is to cope with the control of the air-fuel ratio variation and the torque variation feeling by setting the transition processing time for each change in the air-fuel ratio from “rich” to “lean” and from “lean” to “rich”. 
   (Means for Solving the Problems) 
   To solve these problems, in the present invention, an air-fuel ratio control method of an engine with a venturi type fuel supply device comprises, at least, a venturi chamber located in the upstream of a throttle valve and a passage for supplying air-fuel mixture gas into the venturi chamber. Wherein, basically, the passage is further equipped with a variable air bleeder valve for taking in air. And when the operating state of the external load of the engine changes, the opening of the air bleeder valve is adjusted in accordance with the change so as to control the air-fuel mixture ratio of the mixture gas incoming from the passage into the venturi chamber. 
   When the external load (for example, air-conditioner load and electrical load) changes, the real engine speed changes accordingly from the target engine speed and the negative pressure in the venturi chamber changes. With the above method, however, because the air bleeder valve opening is controlled in accordance with the external load change, the variation range of the air-fuel mixture ratio of the mixture gas incoming from the fuel passage into the venturi chamber can be controlled. Because of this, the present air-fuel ratio in the intake pipe of the engine can be controlled within an allowable variation range even after the change in the load in both cases where the external load increases and decreases, and consequently the engine speed variation resulting from the air-fuel ratio variation can be controlled. Thereby the present invention can prevent ignition failure and restrain deteriorated emission of exhaust gas. 
   Preferably, the air-fuel ratio control method is provided with two or more control variables for adjusting the air bleeder valve opening in accordance with a change in the operating state of the external load and the opening of the air bleeder valve is adjusted by switching the two or more control variables. By providing a table for these control variables, the control method of adjusting the air bleeder valve opening can be simplified. 
   In a preferred mode of the invention, the air-fuel ratio control method is further provided with a transition processing for adjusting the air bleeder valve opening, the opening is adjusted gradually, and the transition quantity and the transition time of the air bleeder valve on the occasion of switching from “there is no external load” (namely externally “Not loaded”) to “there is an external load” (namely externally “Loaded”) condition are set differently from those on the occasion of switching from externally “Loaded ” to “Not loaded” condition. 
   With this mode, a driver&#39;s feeling of torque variation can be restrained. Besides, in an engine using gas fuel, the ignition failure limit on the “lean” side is generally higher than that on the “rich” side. For this reason, if the transition quantity and the transition time of the air bleeder valve on the occasion of switching from externally “Not loaded” to “Loaded” condition are set, for example, less than those on the occasion of switching from externally “Loaded” to “Not loaded” condition, it can cope with the control of the air-fuel ratio variation and the torque variation feeling. 
   In another mode of the present invention, the venturi type fuel supply device further comprises a bypass passage bypassing the throttle valve and a bypass valve (ex. ISC valve) installed in the bypass passage, the bypass valve opening is adjusted in the case of a change in the operating state of the external load, and the air bleeder valve opening is adjusted in accordance with the adjustment quantity of the bypass valve opening. 
   With this method, the bypass valve (ISC valve) opening is adjusted, using a suitable control means, in accordance with a change in the external load so as to adjust the required idling air flow volume (mixture air flow volume) higher or lower. And also the air bleeder valve opening is adjusted in accordance with the consequent pressure change in the venturi chamber. As a result, the variation range of the air-fuel mixture ratio of the mixture gas incoming from the fuel passage into the venturi chamber can be controlled in accordance with the required idling air flow volume (mixture air flow volume), and hence the engine speed variation resulting from the air-fuel ratio variation can be surely controlled. 
   The present invention also discloses a fuel control appliance including the above-mentioned air-fuel ratio control method. The fuel control appliance comprises, at least, a venturi chamber located in the upstream of a throttle valve of an engine, a passage for supplying air-fuel mixture gas into the venturi chamber, a variable air bleeder valve, installed in the passage, for taking in air, a detection means for detecting the operating state of the external load of the engine, a control means that obtains control variables for adjusting the air bleeder valve opening, when the operating state of the external load of the engine changed, based on the detected operating state of the external load, and an air bleeder valve adjustment means for adjusting the opening of the air bleeder valve in accordance with the control variables so as to control an air-fuel ratio of the mixture gas incoming from the passage into the venturi chamber. 
   Preferably, the control means obtains two or more control variables in accordance with the information from the detection means, and the air bleeder valve adjustment means operates by switching the two or more control variable. 
   In another mode of the invention, the fuel control appliance further comprises a bypass passage bypassing the bypass valve (ex. ISC valve), a bypass valve installed in the bypass passage, and a bypass valve adjustment means for adjusting the bypass valve opening based on the change of the operating state of the external load. And the air bleeder valve adjustment control means adjusts the air bleeder valve opening in accordance with the adjustment quantity of the bypass valve opening. 
   The operation of the fuel control appliance according to the present invention is similar to that of the afore-mentioned air-fuel ratio control method of the engine with the venturi type fuel supply device. 
   The method and appliance according to the present invention turn to be very much functional in the case of an engine using gas fuel, such as CNG, as its main fuel but they are applicable also to a gasoline engine and to an engine using both gas and gasoline by switching. Beside, an operation mode of controlling the air bleeder valve opening in accordance with a change in the external load, such as air-conditioner load and electrical load, exhibits effective function particularly in the case the engine is on idling, but, in a practical sense, it naturally can produce similar effect even on non-idling. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an example control block diagram of the fuel control appliance of the present invention. 
       FIG. 2  is an example construction of an engine and its surroundings which the fuel control appliance of the present invention controls. 
       FIG. 3  is an example internal configuration of the fuel control appliance of the present invention. 
       FIG. 4  is an example construction of the venturi chamber and its surroundings of the present invention. 
       FIG. 5  is an example calculation block diagram of the air bleeder opening of the present invention. 
       FIG. 6  is a detailed example of the basic air bleeder opening calculation block of the present invention. 
       FIG. 7  is adetailed example of the load judgment block of the present invention. 
       FIG. 8  is another detailed example of the load judgment block of the present invention. 
       FIG. 9  is an example chart of the transition processing of the air bleeder opening of the present invention. 
       FIG. 10  is another example chart of the transition processing of the air bleeder opening of the present invention. 
       FIG. 11  is an example block diagram for setting the attenuation quantity and attenuation time for the transition processing of the present invention. 
       FIG. 12  is an example operation chart of the air bleeder opening to which the present invention applies. 
       FIG. 13  is an example chart of the engine speed and air-fuel ratio behavior of the present invention. 
       FIG. 14  is another example chart of the engine speed and air-fuel ratio behavior of the present invention. 
       FIG. 15  is an example control flowchart of the fuel control appliance including the air-fuel ratio control method with the venturi type fuel supply device of the present invention. 
       FIG. 16  is an example overall flowchart of the air bleeder opening calculation block of the present invention. 
       FIG. 17  is an example detailed flowchart of the calculation block of the basic air bleeder opening of the present invention. 
       FIG. 18  is an example flowchart of the load judgment block of the present invention. 
       FIG. 19  is another example flowchart of the load judgment block of the present invention. 
       FIG. 20  is an example detailed flowchart for setting the attenuation quantity and attenuation time for the transition processing of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   (Description of the Preferred Embodiments) 
   The preferred embodiments of the present invention are described hereunder, using the attached drawings. It goes without saying that the present invention is not limited to the embodiments described hereunder. 
     FIG. 1  is an example of a control block diagram of a fuel control appliance including the air-fuel ratio control method of a venturi type fuel supply device to which the present invention applies.  FIG. 2  shows an example of a construction of an engine and its surroundings which the fuel control appliance of the present invention controls. The control blocks in  FIG. 1  are explained hereunder, making reference also to FIG.  2 . 
   In  FIG. 1 , a block  101  is an engine speed calculation means. The block counts and computes the electrical signals, mainly the number of inputs per unit time in the pulse signal changes from the cam (crank) angle sensor  209  installed at a specified cam (crank) angle on the engine  201  so as to calculate the engine  201  speed per unit time. 
   A block  102  processes the electrical signal of the opening of the throttle valve  202  and judges idling/non-idling of the engine  201 . 
   A block  103  specifies a target engine speed of the engine  201  on idling by using the speed of the engine  201  computed in the block  101 , engine load, external load such as air-conditioner load, and engine water temperature. And then the block  103  determines the opening of the ISC valve (bypass valve)  205  through feedback control so that the specified engine speed is attained. The block  103  has also a means for judging a change in the external load of the engine  201  based on a change of required air flow rate (ISCQA; ISC air quantity) of the ISC valve  205 . 
   A block  104  is inputted the speed of the engine  201  computed by the block  101  and the pressure of an intake pipe detected, as engine load, by the pressure sensor  206  which is located in the intake pipe  204  of the engine  201 . The block  104  calculates the basic opening of the air bleeder valve  208  based on the engine speed and the intake pressure so that the air-fuel ratio for the engine  201  becomes optimum in each engine driving area. With the basic opening of the air bleeder valve  208  calculated as above, the block  104  processes the basic opening transition, corrects the basic opening, corrects the feedback control correction coefficient through air-fuel ratio feedback control, learns the air-fuel ratio correction coefficient, and applies the learnt value, all of which are to be described later, and then outputs the result as the air bleeder valve opening. The block  104  is also provided with another means for correcting the opening for the start-up of the engine  201 . 
   Using the above engine speed, above engine load, engine water temperature, and output from the oxygen density sensor  212  located in an exhaust pipe of the engine  201 , a block  105  calculates the air-fuel ratio feedback control coefficient so that the air-fuel mixture gas supplied to the engine  201  is kept at the target air-fuel ratio, to be described later. The oxygen density sensor  212  shown in  FIG. 2  is a type that outputs a proportional signal for the exhaust air-fuel ratio, but another type that outputs two signals from the exhaust gas, namely “rich” side signal and “lean” side signal on the basis of theoretical air-fuel ratio, is also acceptable. 
   A block  106  determines the optimum ignition timing in each driving area of the engine  201  by searching into a map or the like, using the above engine speed, above engine load, and engine water temperature. 
   A block  107  calculates a learnt opening of the air bleeder valve  208 , which is correspond to a deviation from the target air-fuel ratio, by using the air-fuel ratio feedback control coefficient calculated in the block  105 , and stores the calculated result as the learnt opening. 
   A block  108  controls the actual opening (air bleeder opening) of the air bleeder valve  208  by using the air bleeder valve opening calculated in the block  104 . 
   A block  109  controls the actual opening of the ISC valve  205  by using the ISC valve opening determined through feedback control in the block  103 . 
   A block  100  is an ignition means for igniting the air-fuel mixture gas incoming into the cylinder according to the ignition timing determined in the block  106 . In this embodiment, the engine load is represented by the pressure of the intake pipe  204  which is measured with the pressure sensor  206 . But it may be represented by the intake air flow rate let into the engine  201 . 
   In an example construction of an engine and its surroundings shown in  FIG. 2 , the engine  201  and its surroundings comprise a throttle valve  202  for controlling the intake air flow rate, a choke valve  203 , of which opening is adjusted by a mechanical linkage with the throttle valve, in the upstream of the throttle valve  202 , a bypass passage  205   a  connected to the intake pipe  204  bypassing the throttle valve  202 , an ISC valve  205  for controlling the flow area of the bypass passage and controlling the engine speed on idling, an intake pipe pressure sensor  206  for detecting the pressure in the intake pipe  204 , a regulator  207  for regulating the pressure of fuel (for example, CNG) supplied to the engine  201 , an air bleeder valve  208  which is located in the downstream of the regulator  207  and controls the flow area of the passage set open to the atmosphere, a cam (crank) angle sensor  209  installed at a specified position on the engine  201 , an ignition module  210  for supplying ignition energy, based on the ignition signal from the engine control appliance  215 , to the spark plug that ignites the air-fuel mixture gas supplied into the cylinder of the engine  201 , a water temperature sensor  211 , which is installed in the cylinder block of the engine  201 , for detecting the cooling water temperature of the engine  201 , an oxygen density sensor  212 , which is installed in the exhaust pipe of the engine  201 , for detecting the oxygen density in the exhaust gas, an ignition key switch  213  as the main start/stop switch of the engine, an air-conditioner switch  214  for turning ON/OFF the air-conditioner, an engine control appliance  215  for controlling the air-fuel ratio and ignition of the engine  201 , an electrical load switch (not shown) for turning ON/OFF the electrical systems of the vehicle, and so on. The oxygen density sensor  212  shown in  FIG. 2  is a type that outputs a proportional signal for the exhaust air-fuel ratio. But, as explained before, it is also acceptable another type that outputs two signals from the exhaust gas, such as “rich” side signal and “lean” side signal on the basis of the theoretical air-fuel ratio. Besides, in this embodiment, the fuel control is performed by detecting the pressure of the intake pipe  204 , but it also is possible that the air-fuel ratio control is performed by detecting the intake air flow rate let into the engine  201 . 
     FIG. 3  shows an example of the internal configuration of a fuel control appliance including the air-fuel ratio control method of a venturi type fuel supply device to which the present invention applies. The appliance comprises an I/O driver  301 , a main processing unit (MPU)  302 , a non-volatile memory (EP-ROM)  303 , and a volatile memory (RAM)  304 . 
   The I/O driver  301  converts the electrical signal from each sensor installed in the engine to a signal for digital computation, and also converts the control signal for digital computation to an actual actuator drive signal. 
   The main processing unit (MPU)  302  judges the engine condition from the digital computation signals from the I/O driver  301 , and calculates the fuel quantity, ignition timing, etc. required by the engine, based on programmed procedure, and then sends the calculation result to the I/O driver  301 . 
   The non-volatile memory (EP-ROM)  303  stores control protocols and control constants of the processing unit (MPU)  302 . The volatile memory (RAM)  304  stores the calculation result of MPU  302 . A backup power supply may be connected to the volatile memory (RAM)  304  so that the stored memory is held even in the case the ignition key switch  213  is OFF and no power is supplied to the fuel control appliance  215 . 
   In this embodiment, e.g. various signals from the water temperature sensor  211 , the crank angle sensor  209 , the oxygen density sensor  212 , the intake pipe pressure sensor  206 , the throttle opening sensor  202 , the ignition switch  213 , the air-conditioner switch  214 , and the electrical load switch are inputted to the fuel control appliance. And the opening instruction values  313  to  316  of the air bleeder valve  208 , the opening instruction values  317  to  320  of the ISC valve  205 , the ignition signal  321 , and the valve drive signal  322  of the regulator  207  are outputted from the fuel control appliance. 
     FIG. 4  shows an example of the construction of the venturi chamber  400  and its surroundings between the choke valve  203  and the throttle valve  202  of a venturi type fuel supply device to which the present invention applies. The choke valve  203  is connected to the throttle valve  202  with a mechanical linkage  403 . A passage  401 , in which the air bleeder valve  208  for determining the air and fuel gas mixture ratio of the mixture gas is installed, connects to the venturi chamber  400 . At the time of an idling, the mechanical linkage  403  is operated so that a negative pressure necessary for taking the mixture gas from the passage  401  is generated in the venturi chamber  400 . In addition, a passage (bypass passage)  205   a , which has the flow area controlled by the ISC valve  205 , is provided bypassing the throttle valve  202 . With this construction, when the ISC valve  205  is opened, the venturi pressure Pb shown in the figure decreases as it is drawn by the pressure Pm in the intake pipe  204 , and accordingly the air-fuel ratio of the mixture gas incoming from the passage  401  changes even if the air bleeder valve  208  opening remains the same. The air-fuel ratio tends to become “rich” if the ISC valve  205  is opened and “lean” if it is closed. The subject matter of the present invention is to minimize the air-fuel ratio variation by controlling the opening of the air bleeder valve  208 . 
     FIG. 5  is an example of a calculation block diagram of the air bleeder opening to which the present invention applies. A block  501  calculates the basic air bleeder opening from the detected engine speed, engine load, external load such as electrical load and air-conditioner load, and idling judgment result. A block  502  calculates a correction rate of the air bleeder opening correspond to the engine speed compensation from the engine speed, external load and engine water temperature. A block  503  calculates a correction rate of the air bleeder opening correspond to the water temperature compensation from the engine water temperature. These correction rates are added in an adder  504  and calculated as the air bleeder opening before complete explosion. Either the basic air bleeder opening or the air bleeder opening before complete explosion is selected, by a switch  505 , according to the complete explosion judgment in block  506 , and the selected one is outputted as the air bleeder opening. In this example, complete explosion is judged from the engine speed after start-up. 
     FIG. 6  is a detailed example of the basic air bleeder opening calculation block  501  shown in FIG.  5 . Each block  601  and  602  is a map for searching the basic air bleeder opening on non-idling. A block  601  is a map to be used when the external load is judged OFF, and a block  602  is a map to be used when the external load is judged ON. The air bleeder opening is searched in each map, using the engine speed and engine load. Each block  603  and  604  is a table for searching the basic air bleeder opening on idling. A block  603  is a table to be used when the external load is judged OFF, and a block  604  is a table to be used when the external load is judged ON. The air bleeder opening is searched in each table  603  and  604 , using the engine water temperature. Each block  605  and  606  is for transition processing that is necessitated when the maps or tables are changed over depending upon the external load ON/OFF. The external load ON/OFF judgment is made, based on the load judgment value in block  607 , the air conditioner switch and the electrical load switch. In this embodiment, the external load is judged ON by using an OR circuit in block  608 ,if any one of the load judgment value, air-conditioner switch and electrical load switch is ON (the load judgment value is “loaded”). The transition processing to be necessitated in changing over idling/non-idling each other is performed in a block  609 . The idling/non-idling judgment is processed in a block  610 , using the throttle valve opening. The air bleeder opening processed in block  609  is outputted as the basic air bleeder opening. The basic air bleeder opening is changed over between two maps, depending upon the external load ON/OFF, in this embodiment, but another map based on different factor may be added. 
     FIG. 7  is a detailed example of the load judgment block  607  shown in FIG.  6 . The differentiator  701  calculates the difference between the present engine speed and target engine speed. Based on the difference, the feedback control variables of the required ISC air quantity (ISCQA) are calculated in blocks  702 ,  703  and  704 . A block  702  calculates a P component of the feedback control, a block  703  calculates an I component, block  704  calculates a D component, and the adder in block  705  adds up the P component, I component and D component, the result of which is the feedback control variable ISCFB. A block  706  is for searching the basic quantity of the ISC air quantity (ISCQA) by using a table. The basic quantity is searched into the table, using the engine water temperature. The basic quantity searched in block  706  is added to the feedback control variable (ISCFB) in the adder  707  and outputted as the ISC air quantity (ISCQA). A block  708  is for searching the basic feedback control variable. It is searched in a similar manner as for above basic quantity by using the engine water temperature. The basic feedback control variable searched in block  708  is compared with the feedback control variable ISCFB in the comparator  709 , and, if the feedback control variable ISCFB is greater, a load judgment value meaning “Loaded” is outputted to the block  608 . 
     FIG. 8  is another detailed example of the load judgment block shown in FIG.  6 . It differs from the example in  FIG. 7  in a point that multiple tables are provided for searching the basic quantity of the ISC air quantity (ISCQA) in a block  806 . The multiple tables are changed over each other by a switch  811  if either the electrical load or the air-conditioner switch signal is inputted through the OR circuit in a block  810 . In a block  808 , the basic ISC air quantity (ISCQA) is searched, using the engine water temperature, and the result is compared with the ISC air quantity (ISCQA) in the comparator  809 . If the ISC air quantity (ISCQA) is greater than the basic ISCQA, a load judgment value meaning “Loaded” is outputted. 
     FIG. 9  is an example chart of the transition processing of the air bleeder opening to which the present invention applies. When the condition changes from “Loaded (there is an external load)” to “Not loaded (there is no external load)” in chart  901 , the air bleeder opening shown in chart  902  converges to the final ultimate opening  903  with the passing of an attenuation quantity  904  and an attenuation time  905 . The convergence time  906  up to the final ultimate opening  903  is T open .  FIG. 10  is another example chart of the transition processing of the air bleeder opening to which the present invention applies. While  FIG. 9  shows an occasion of changing from “Loaded” to “Not loaded” condition, this example shows an occasion of changing from “Not loaded” to “Loaded” condition. When the condition changes from “Not loaded” to “Loaded” in chart  1001 , the opening converges to the final ultimate opening  1005  in a time T close    1006 , in a similar manner as in the example in FIG.  9 . This convergence time to the final ultimate opening and that of the example in  FIG. 9  have a relationship expressed by Formula (1) below.
 T open ≦T close   (1) 
   That is, the convergence time up to ultimate opened position of the air bleeder valve is set shorter than that up to ultimate closed position. The attenuation (convergence) time and attenuation quantity, however, can be set freely depending upon the operating condition of the engine. 
     FIG. 11  is an example block diagram for setting the attenuation quantity and the attenuation time for the transition processing in FIG.  9  and FIG.  10 . Each block  1101  and block  1102  determines the attenuation quantity for the transition processing by using table search. Block  1101  searches the attenuation quantity under “Loaded (there is an external load)” condition and block  1102  searches the attenuation quantity under “Not loaded (there is no external load)” condition into respective tables, using the engine water temperature. Each block  1103  and  1104  determines the attenuation time for the transition processing by using table search. A block  1103  searches the attenuation time under “Loaded (there is an external load)” condition and block  1104  searches the attenuation time under externally “Not loaded (there is no external load)” condition into respective tables, in a similar manner as for the attenuation quantity, using the engine water temperature. The air-conditioner switch signal, load judgment value from the load judgment block, and electrical load switch signal are inputted into the OR circuit in block  1105 , and the attenuation quantity and attenuation time under each externally “Loaded”/“Not loaded” condition is changed over by switches  1106  and  1107 . 
     FIG. 12  is an example operation chart of the air bleeder opening to which the present invention applies. A chart  1201  represents the electrical load switch, a chart  1202  represents the air-conditioner switch, a chart  1203  represents the ISCQA, a chart  1204  represents the load judgment value, and a chart  1205  represent the air bleeder opening. Even when the electrical load switch is turned ON at the timing  1206  and ISCQA in chart  1203  increases, the load judgment value in chart  1204  makes a judgment of externally “Not loaded” because the ISCQA does not exceeds the basic ISCQA  1208 . As the air-conditioner switch is turned ON at the timing  1207 , the ISCQA in chart  1203  further increases and exceeds the basic ISCQA  1208 . As a result, the load judgment value changes a judgment from externally “Not loaded” to “Loaded” and the transition processing of the air bleeder opening in chart  1205  begins. 
     FIG. 13  is an example chart of the engine speed and air-fuel ratio behavior in a venturi type fuel supply device including the air-fuel control method to which the present invention applies. A chart  1301  represents the load judgment value, a chart  1302  represents the air bleeder opening, a chart  1303  represents the negative pressure (Pb) in the venturi chamber  400 , a chart  1304  represents air-fuel ratio, and a chart  1305  represents the engine speed. This embodiment is an example where the external load such as air-conditioner is applied and the ISCQA increases, and consequently the load judgment value is given a judgment of externally “Loaded”. Because the ISCQA increases (the ISC valve  205  is made open), the venturi negative pressure (Pb) in chart  1303  becomes lower than the condition of “Not loaded”. In this case, the transition processing of the air bleeder opening in chart  1402  is performed from the closed position side to the opened position side. Because of this, in the area shown in chart  1304 , the air-fuel ratio variation shown by a solid line, which represents a case where the air bleeder opening changeover and attenuation processing of the present invention are applied, is smaller on the “rich” side than that shown by a dotted line which represents a case where the above are not applied. For the same reason, the engine speed in chart  1305  shown by a dotted line, which represents a case where the present invention is not applied, has decreased due to “rich” ignition failure in the area  1308 , but that shown by a solid line, which represents a case where the present invention is applied, does not decrease. 
     FIG. 14  is another example chart of the engine speed and air-fuel ratio behavior in a venturi type fuel supply device including the air-fuel control method to which the present invention applies. This chart differs from the chart in the previous  FIG. 13  in a point that this shows a case where the external such as air-conditioner is turned OFF. In this case, the transition processing of the air bleeder opening in chart  1402  is performed from the opened position side to the closed position side. The air-fuel ratio in chart  1404  shown by a dotted line, which represents a case where the present invention is not applied, exhibits a big variation on the “lean” side. On the other hand, a gas fuel engine according to the present invention has higher ignition failure limit on the “lean” side than on the “rich” side. Because of this, even in the case shown by a dotted line to which the present invention is not applied, the engine can recover from decreased revolution due to ignition failure as shown in the area  1408  although the convergence time is longer than the case in FIG.  13 . 
     FIG. 15  is an example control flowchart of a fuel control appliance including the engine air-fuel ratio control method with a venturi type fuel supply device to which the present invention applies. A block  1501  calculates the engine speed, and a block  1502  reads the engine load such as the intake pipe pressure. A block  1504  reads the engine water temperature, and a block  1505  calculates the basic ignition timing based on the engine speed, engine load and engine water temperature obtained above. A block  1506  sets the ISC target engine speed based on the obtained engine water temperature, and a block  1507  performs feedback control so that the engine speed is set to the ISC target engine speed. In a block  1508 , the control variable obtained through the ISC feedback control is outputted to the ISC valve. A block  1509  reads the oxygen density sensor output, and a block  1510  performs air-fuel ratio feedback control. After the air-fuel ratio feedback control is complete, a block  1511  calculates the learnt air bleeder opening and stores (records into memory) the learnt opening, using the air-fuel feedback control variable obtained above. A block  1512  judges complete explosion or not of the engine, based for example on the engine speed. If the engine is judged not complete explosion condition, a block  1513  calculates the start-up air bleeder opening. If the engine is judged complete explosion condition in the block  1512 , the blocks 1514  to  1516  are processed. Block  1514  calculates the basic air bleeder opening, using the engine speed and engine load. A block  1515  performs transition processing of the basic air bleeder opening. A block  1516  corrects the basic opening learnt opening obtained from air-fuel ratio learning. A block  1517  outputs the instruction value of the air bleeder opening, calculated above, as the air bleeder opening. 
     FIG. 16  is an example overall flowchart of the air bleeder opening calculation block in FIG.  5 . In this embodiment, a flow chart of consecutive calculation blocks of the air bleeder opening around the start-up of the engine. A block  1601  reads the engine speed. A block  1602  reads the engine load. A block  1603  judges complete explosion or not of the engine and, if judged complete explosion condition, a block  1604  searches the basic air bleeder opening into a map. If the engine is judged not complete explosion in the block  1603 , blocks  1605 ,  1606 ,  1607  and  1608  search the engine speed based correction quantity and water temperature based correction quantity into tables and add the results, and the sum of the results is the basic air bleeder opening. A block  1609  outputs the basic air bleeder opening, corresponding to the complete explosion/not complete explosion condition. 
     FIG. 17  is an example detailed flowchart of the calculation block of the basic air bleeder opening. A block  1701  reads the engine speed. A block  1702  reads the engine load. A block  1703  reads the throttle opening, and a block  1704  judges idling/non-idling. A block  1705  judges external load shown in  FIGS. 18 and 19 , to be described later. A block  1706  judges idling/non-idling. When judged idling, blocks  1707  to  1713  are processed. The block  1707  judges whether the external load is OFF. When the external load is judged “OFF”, the block  1708  searches the basic air bleeder opening under “external load OFF” into a table, using the engine water temperature. The block  1709  judges whether the transition processing is complete. If the transition processing is not complete, the block  1710  performs the transition processing. If the external load is judged “ON” in the block  1707 , the blocks  1711  to  1713  are processed in a similar manner as when judged OFF. If the engine is judged “non-idling” in the block  1706 , the blocks  1714  to  1720  are processed in a similar manner as when judged “idling”. The basic air bleeder opening when judged “non-idling” is searched into a map, using the engine speed and engine load. In this embodiment, the transition processing is determined complete if the present air bleeder opening has reached the specified final value. 
     FIG. 18  is an example flowchart of the load judgment block in  FIG. 7. A  block  1801  reads the present engine speed and the target ISC engine speed, and a block  1802  calculates the difference between the engine speed and the target engine speed obtained above. Blocks  1803  to  1805  calculates the P, I, D components of the ISC feedback control, respectively and a block  1806  adds up them and calculates the feedback control variable ISCFB. A block  1807  reads the engine water temperature, and a block  1808  searches the ISC air quantity into a table, using the engine water temperature obtained above. The ISC air quantity searched from the table is added to the feedback control variable ISCFB in a block  1809  and the ISC opening is determined. A block  1810  searches the basic ISCFB into a table, using the engine water temperature. The basic ISCFB searched from the table is compared with the feedback control variable ISCFB in a block  1811  and a block  1812 . If the feedback control variable ISCFB is greater, a block  1813  judges externally “Loaded”. If the feedback control variable ISCFB is smaller, a block  1814  releases the load judgment. 
     FIG. 19  is an example flowchart of the load judgment block in FIG.  8 . It is almost similar to the flowchart in FIG.  18 . But it differs only in a point that a block  1909  selects suitable ISC air quantity table for each load switch (air-conditioner switch, electrical switch, etc.) and that a block  1912  searches the basic ISCQA, using the engine water temperature, and compares the basic ISCQA with ISCQA (in blocks  1913  and  1914 ) to judge the external load. 
     FIG. 20  is an example detailed flowchart for setting the attenuation quantity and the attenuation time for the transition processing in  FIG. 11. A  block  2001  reads the engine water temperature. Blocks  2002  to  2005  search respective attenuation quantity and attenuation time into tables, using the engine water temperature obtained above. A block  2006  reads the air-conditioner switch, electrical load switch, etc. A block  2007  reads the external load judgment value. A block  2008  judges whether loaded by any of them and, if any, a block  2009  selects the attenuation time and the attenuation quantity under externally “Loaded”. If judged no load, a block  2010  selects the attenuation time and attenuation quantity under externally “Not loaded”. 
   (Effects of the Invention) 
   According to the present invention, with a venturi type fuel supply device under external load variation, the air-fuel mixture can be supplied without changing the air-fuel ratio greatly so as to maintain the engine speed corresponding to the external load variation. In a preferable mode of the invention, air-fuel ratio change corresponding to the change in the required ISC air quantity at the time of external load variation can be corrected, using the air bleeder opening. Because of this, ignition failure due to idling variation or engine speed variation resulting from the air-fuel ratio variation is not caused. Besides, because the air-fuel ratio variation can be controlled, the deterioration of exhaust gas emissions can be restrained. 
   In another preferable mode of the invention, the transition processing is provided so as to control the air-fuel ratio variation, and hence a driver&#39;s feeling of torque variation can be restrained. Besides, the transition processing time is set separately for each air-fuel ratio change from “rich” to “lean” and from “lean” to “rich”, and hence it can cope with the control of the air-fuel ratio variation and the control of the torque variation feeling.