Air-fuel control method and apparatus for internal combustion engine

An air-fuel ratio control apparatus for an internal combustion engine comprising:

• a carburetor including a low speed fuel system and a main fuel system and serving for controlling the air-fuel ratio of a fuel-air mixture supplied to said internation combustion engine;
• a thermal reactor in which exhaust gas from said internal combustion engine is reacted with secondary air supplied to said reactor;
• a three-way catalyst tube connected to said thermal reactor;
• an exhaust gas composition sensor disposed in an exhaust gas passage between said thermal reactor and said three-way catalyst tube;
• a control circuit for producing a control pulse signal having a pulse width corresponding to the output signal from said sensor;
• a negative pressure generator for producing a negative pressure in response to said control pulse signal from said control circuit;
• a detector for detecting a low speed operation region and a high speed operation region of said internal combustion engine and adapted to produce an output signal which is applied to said control circuit to alter the level of said control signal so that said internal combustion engine is supplied with a fuel-air mixture of a low air-fuel ratio when said region detector indicates that said engine is operating in the low speed region, while said engine is supplied with a fuel-air mixture of an approximately stoichiometric air-fuel ratio when said region detector indicates that said engine is operating in the high speed region;
• means for controlling said low speed fuel system and said main fuel system, respectively, in response to the negative pressure produced by said negative pressure generator;
• and means for controlling said second air supply in response to the negative pressure produced by said negative pressure generator in such manner that the secondary air supply to said thermal reactor is increased when said operation region detector indicates that said engine is operating in said low speed region, while a constant and small quantity of secondary air is supplied to said thermal reactor when said operation region detector indicates that said engine is operating in a high speed region.

A method of controlling an air-fuel ratio control apparatus for an internal combustion engine which comprises a carburetor including a low speed fuel system and a main fuel system and adapted to control the air-fuel ratio of a fuel-air mixture supplied to said engine, a thermal reactor in which exhaust gas discharged from said engine is caused to react with secondary air supplied to said reactor, an exhaust gas composition sensor disposed in a discharge passage extending from said thermal reactor, a negative pressure generator for producing a negative pressure in response to the output signal from said exhaust gas composition sensor, and a detector for detecting a low speed and a high speed rotation region of said internal combustion engine;

• comprising the steps of:
• detecting the air number (excess-air ratio) of said exhaust gas by means of said exhaust gas composition sensor;
• producing a control pulse signal having a pulse width varying in correspondence to the detected air number (excess air ratio) and varying the pulse width stepwise in response to the output signal from said operation region detector;
• varying the negative pressure produced by said negative pressure generator in accordance with said control pulse signal;
• controlling said low speed fuel system and said main fuel system in response to the negative pressure produced by said negative pressure generator in such a manner that the fuel-air mixture of low air-fuel ratio is supplied to said engine when said operation region detector indicates that said engine is operating in the low speed region, while the fuel-air mixture having an approximately stoichiometric air-fuel ratio is supplied to said engine when said operation region detector indicates that said engine is operating in the high speed region;
• and
• controlling the quantity of said secondary air supply in response to the negative pressure produced by said negative pressure generator in such a manner that said secondary air supply to said thermal reactor is increased when said operation region detector indicates that said engine is operating in the low speed region, while a constant and smaller quantity of secondary air is supplied to said thermal reactor when said operation region detector indicates that said engine is operating in the high speed operation region;
• wherein the fuel-air mixture having a lower air-fuel ratio than the stoichiometric air-fuel ratio is supplied to said engine with the secondary air supply to said thermal reactor being concurrently increased when said engine is operating in the low speed operation region, while in the high speed operation region of said engine the fuel-air mixture having the approximately stoichiometric air-fuel ratio is supplied to said engine with the secondary air supply to said thermal reactor being controllably adjusted to a constant and smaller quantity.