Air fuel ratio controller
BACKGROUND OF THE INVENTION 
     The present invention relates to an air-fuel ratio controller having a feedback system in which the air-fuel ratio of the mixture formed in the carburetor is controlled in accordance with a signal derived from an exhaust gas sensor and, more particularly, to an improvement in an actuator for use in the feedback system and adapted for effecting the control of the air-fuel ratio. 
     There have been proposed and actually used various types of air-fuel ratio controllers of the kind described. In a typical conventional air-fuel ratio controller, as will be explained later in more detail, a passage communicating with the air bleed of the main system and a passage communicating with the air bleed of the slow system of the carburetor are opened in a common valve seat which is flat and adapted to make a surface contact with a cooperating flat valve. Therefore, if the valve is mounted at a slight inclination or if the valve seat is not sufficiently finished, the passages leading to respective air bleeds often communicate with each other, even when the valve is seated on the valve seat. In such a case, since the vacuum in the slow system is greater than the vacuum established in the main system, the fuel is inconveniently induced from the main system into the slow system. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to overcome the above described problem of the prior art by providing an improved air-fuel ratio controller. 
     To this end, according to the invention, there is provided an air-fuel ratio controller of a type having an exhaust gas sensor and a controlling circuit in which the flow rates of the air bleeds of the slow and main systems of a carburetor are simultaneously controlled by means of one ON-OFF type electromagnetic valve, characterized by a recess formed in the valve or the valve seat of the solenoid valve at a portion between the openings of the passages which leads to respective air bleeds of the slow and main systems, the recess being adapted to prevent the openings of both passages from communicating with each other, and communicating with a chamber which in turn communicates with the inlet passage for air. 
     The above and other objects, as well as advantageous features of the invention will become more clear from the following description of the preferred embodiments taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a conventional air-fuel ratio controller, 
     FIG. 2 is a sectional view of an essential part of an air-fuel controller constructed in accordance with an embodiment of the invention, and 
     FIG. 3 is a sectional view of an essential part of an air-fuel ratio controller constructed in accordance with another embodiment of the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before turning to the description of the preferred embodiments, an explanation will be made as to the drawback of the conventional air-fuel ratio controller, with specific reference to FIG. 1, so that the advantage of the invention may be more readily and clearly understood. 
     Referring to FIG. 1 showing a conventional air-fuel ratio controller, an intake manifold 1 and an exhaust manifold 2 are attached to an internal combustion engine E. An exhaust gas sensor 3 is disposed in the exhaust manifold 2, while a carburetor body 4 is attached to the upstream side of the intake manifold 1. The carburetor has a large venturi 5, a small venturi 6, a nozzle port 7, a throttle valve 8, an air inlet 9 for air bleeds, an air bleed passage 10 for the main system, and an air bleed passage 11 for the slow system. A control circuit 12 is provided while batteries 12 constitute a power supply. An actuator 14 for actuating the valve for controlling the air supply to the air bleeds includes a casing 15 accommodating a coil bobbin 16 which is adapted to electromagnetically drive an armature 18. A spring 17 is adapted to bias the armature 18 towards a flat valve seat 23 and a valve 19 is attached to the armature 18. An air passage 20 leading from the air inlet 9 opens in the casing 15. Passages 21 and 22 respectively, conduct the air bleed of the main system and the air bleed of the slow system. These passages 21, 22 open in the flat valve seat 23 which is adapted to cooperate with the valve 19. 
     In the conventional air-fuel ratio controller having the described construction, since the air-tightness depends upon the surface contact of the flat valve seat 23 and the flat valve 19 with each other, the passages 21 and 22 inconveniently communicate with each other, even when the valve 19 is seated on the cooperating valve seat 23, if the valve 19 is mounted at a slight inclination or if the valve seat 23 has been finished in a poor manner. In such a case, since the level of the vacuum in the passage 22 of the slow system is generally high, the fuel is inconveniently drawn from the passage 10 of the main system into the passage 11 of the slow system. 
     This problem is fairly overcome by the air-fuel ratio controller of the invention, as will be understood from the description of the preferred embodiment of the invention. 
     Referring now to FIG. 2 showing in section an essential part of an embodiment of the invention, the passages 22, 21 leading to the air bleeds of the slow and the main systems open in the valve seat 23, as in the conventional air-fuel controller. However, according to the invention, a recess 24 is formed in the surface of the valve seat 23, at a portion of the latter between the openings of both passages 21, 22. The recess 24 communicates with the chamber 25, through a passage of a cross-section sufficiently larger than that of the opening of the passage 21, 22. 
     When the coil of the bobbin 16 is not energized, the spring 17 biases the armature 18 and, accordingly, the valve 19, so as to press the latter against the valve seat 23. As the coil of the bobbin 16 is energized, a magnetic force is generated to drive the armature 18 downwardly overcoming the force of the spring 17, so that the valve 19 is moved away from the valve seat 23, so as to allow the passages 21, 22 to communicate with the chamber 25. Consequently, the air is drawn into the passages 21, 22 from the inlet passage 20, through the gap between the valve 19 and the valve seat 23, so as to control the air bleed flow rates in the slow and main systems. The ratio of the opening period to the closing period of the On-Off type electromagnetic or solenoid valve, i.e. the so-called duty ratio, is controlled in accordance with the signal derived from the exhaust gas sensor 3, so that the flow rates of the air bleeds are adjusted to provide the desired air-fuel ratio of the mixture formed in the carburetor. 
     A slight gap may be formed between the valve 19 and the valve seat 23, due to an inclination of the valve 19 or the seat 23, or the insufficient finishing or flatness of the valve seat, even when the valve 19 is seated on the valve seat 23. However, even when such a slight gap is formed, the mutual communication of the passages 21 and 22 is avoided, because the passage 21 or the passage 22 here communicates with the chamber 25, due to the presence of the recess 24 in accordance with the present invention. 
     FIG. 3 shows another embodiment of the invention, in which the recess 24 is formed in the surface of the valve 19, instead of in the valve seat 23. It will be clear to those of ordinary skill in the art that this second embodiment functions in the same manner as the first embodiment as shown in FIG. 2. 
     Thus, in the air-fuel ratio controller of the invention, the passages 21, 22 communicate with the chamber 25, even when a slight gap is formed between the valve 19 and the valve seat 23 due to inclination, insufficient flatness and bad finishing of the valve or the valve seat. 
     Since the chamber 25 is ample enough and the pressure therein is maintained at the same level as that of the air at the inlet of the carburetor, i.e. substantially at the atmospheric pressure, the pressure in the passage 21 is kept at the same level as the pressure at the carburetor inlet, due to the communication of the passage 21 with the chamber 25 through the recess 25. Also, the pressure in the passage 22 is kept at the same level as the pressure at the carburetor inlet, due to the communication of the passage 22 with the chamber 25 through the recess 24. 
     Thus, the mutual direct communication of the passages 21, 22 through the gap between the valve 19 and the valve seat 23 is prevented, and the aforementioned unfavourable drawing of the fuel from the passage 10 of the main air bleed into the passage 11 of the slow air bleed is fairly avoided. 
     It is remarkable that this practical advantage is brought about by simply forming a recess 24 in the valve seat 23 or the valve 19.