Air-fuel ratio controlling system for an internal combustion engine

An air-fuel ratio controlling system for an internal combustion engine having an intake duct with a throttle valve disposed therein, said system comprising:

• an air valve disposed in said intake duct upstream of said throttle valve to cooperate therewith to define an air pressure chamber therebetween;
• an air valve controlling means responsive to variation in the air pressure in said air pressure chamber normally to maintain a substantially constant air pressure difference across said air valve;
• a fuel circuit having at its downstream end a fuel discharge port open to said intake duct;
• a fuel metering means including means defining a fuel-metering variable orifice disposed in said fuel circuit and means for maintaining a substantially constant fuel pressure difference across said fuel-metering variable orifice;
• said fuel-metering orifice defining means being operatively associated with said air valve so that the fuel-flowing area of said fuel-metering orifice is varied in proportion to the variation in the air-flowing opening area defined between said air valve and said intake duct;
• said air valve controlling means and said fuel metering means being cooperative to control the air-fuel ratio of an air-fuel mixture to be fed into the engine such that the air-fuel ratio is normally kept substantially constant;
• an O.sub.2 sensor for detecting the oxygen content of the engine exhaust gases;
• and
• means responsive to signal from said O.sub.2 sensor to adjust the air-fuel ratio;
• wherein:
• said fuel circuit includes a high pressure fuel source kept at a substantially constant first pressure level higher than the atmospheric pressure and a low pressure fuel source kept at a substantially constant second pressure level higher than the atmospheric pressure but lower than said first pressure level;
• said air valve controlling means includes a cylinder, a piston slidably disposed in said cylinder to cooperate therewith to define a first fuel pressure chamber capable of being communicated with said high and low pressure fuel sources, a link means operatively connecting said piston to said air valve, a bore and a spool slidable in said bore and having a valve portion, said spool being movable in said bore in response to variation in the pressure in said air pressure chamber to cause said valve portion to control the communication between said first fuel pressure chamber and said high and low pressure fuel sources thereby to vary the pressure in said first fuel pressure chamber;
• and
• said O.sub.2 sensor responsive means includes a second fuel pressure chamber defined by the cooperation of said bore and one end of said spool and communicated through a first restriction with said high pressure fuel source, a first fuel passage connecting said second fuel pressure chamber to said low pressure fuel source, and a valve means disposed in said first fuel passage and operative in response to variation in the output of said O.sub.2 sensor to control the flow of the fuel from said second fuel pressure chamber through said first fuel passage to said low pressure fuel source thereby to vary the fuel pressure in said second fuel pressure chamber, the variation in the pressure in said second fuel pressure chamber moving said spool to vary the pressure in said first fuel pressure chamber so that said piston is moved to vary the degree of opening of said air valve and thus vary the air pressure difference across said air valve whereby the air-fuel ratio is adjusted.