Air-fuel ratio control system
BACKGROUND OF THE INVENTION 
     The present invention relates to an air-fuel ratio control system for an internal combustion engine, which system controls the air-fuel mixture to the stoichiometric air-fuel ratio, at which ratio a three-way catalyst acts most effectively. 
     In a known air-fuel ratio control system for a motor vehicle, the air-fuel ratio of the air-fuel mixture burned in the engine cylinders is detected as the oxygen concentration in the exhaust gases by means of an O 2  sensor provided in the exhaust system of the engine, and a decision is made dependent on the output signal from the O 2  sensor which indicates whether the air-fuel ratio is richer or leaner than the value corresponding to the stoichiometric air-fuel ratio, for producing a control signal. The control system is provided with a basic pulse generating section for generating basic pulses, and a calculating section which operates to correct the duty ratio of the basic pulses in accordance with the control signal so as to meet driving conditions. The pulses operate an electromagnetic valve so as to control the amount of bleed air in a carburetor for controlling the air-fuel ratio of the mixture. When the duty ratio of the pulses is reduced, the air-fuel mixture is enriched. Thus, the air-fuel ratio is controlled to the stoichiometric air-fuel ratio, at which a three-way catalyst in the exhaust system acts most effectively. 
     In such an air-fuel ratio control system at cold engine operation, the air-fuel ratio is controlled by open loop control with fixed duty ratios stored in a look-up table. However, the look-up table can not be provided so as to supply an air-fuel mixture having a duty ratio which satisfies both conditions of steady state driving and transient state driving such as acceleration. Although the amount of intake air increases, when the engine is accelerated, the amount of induced fuel does not increase with an increase of the intake air. Accordingly, the air-fuel mixture must be enriched upon acceleration. If the table is made to meet the transient state, the air-fuel ratio is improper for the steady state. 
     SUMMARY OF THE INVENTION 
     Accordingly, the object of the present invention is to provide a system which may effectively control the air-fuel ratio at acceleration of an engine during cold engine operation. 
     Other objects and features of this invention will become understood from the following description with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is a schematic explanatory view of an air-fuel ratio control system according to the present invention; 
     FIG. 2 shows a block diagram of the electric control circuit of the present invention; 
     FIG. 3 shows a look-up table for air-fuel ratio; and 
     FIG. 4 shows a flowchart showing the operation of the system. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, a carburetor 1 is provided adjacent to an intake manifold (induction passage) 21 of an internal combustion engine 2. A correcting air passage 8 communicates with an air-bleed 7 which is provided in a main fuel passage 6 between a float chamber 3 and a nozzle 5 in a venturi 4. Another correcting air passage 13 communicates with another air-bleed 12 which is provided in an idle fuel passage 11 which diverges from the main fuel passage 6 and extends to an idle port 10 in the vicinity of a throttle valve 9. These correcting air passages 8 and 13 communicate with on-off type electromagnetic valves 14, 15, the induction sides of which are in communication with the atmosphere through an air filter 16. A three-way catalytic converter 18 is provided in an exhaust pipe 17 downstream of the engine, and an O 2  sensor 19 is provided between the engine 2 and the converter 18 to detect the oxygen concentration of exhaust gases when the air-fuel mixture is burned in the engine. A coolant temperature sensor 20 is provided on a water jacket of the engine for detecting the temperature of cooling water and a vacuum sensor 22 is provided in the intake manifold 21 downstream of the throttle valve 9, and an atmospheric pressure sensor 23 is provided in the system to detect pressure for correcting the air-fuel ratio. 
     The outputs of the O 2  sensor 19, coolant temperature sensor 20, vacuum sensor 22, and the atmospheric pressure sensor 23 are sent to a control unit 30 which produces an output signal to actuate the electromagnetic valves 14, 15 to open and close them at a duty ratio. Thus, either considerable air is supplied to the fuel system through the air correcting passages 8, 13 to produce a lean air-fuel mixture or only a small amount of air is supplied to the system so as to enrich the air-fuel mixture. 
     FIG. 2 shows the construction of the control unit 30 which includes a feedback control circuit (31, 32, 34, 39, 36). The control unit is provided with a basic pulse generating section 34 for producing basic pulses having a constant duty ratio, which are sent to a calculating section 32. The output of the O 2  sensor 19 is applied to the calculating section 32 through a comparator 31. 
     Generally, the air-fuel ratio varies cyclically with respect to the stoichiometric air-fuel ratio. Accordingly, the output of the O 2  sensor 19 has a waveform having a wavelength. The output is compared with a reference value at the comparator 31 which produces error signal pulses dependent on the waveform. The pulses are applied to the calculating section 32, where the basic pulses supplied from the section 34 is corrected by the error signal pulses to generate controlled or corrected output pulses, the duty ratio of which is corrected to correct the deviation of the air-fuel ratio. The controlled output pulses are supplied to the electromagnetic valves 14, 15 via a changeover circuit 39 and a driver 36 for operating the valves. 
     When a rich air-fuel mixture is detected, the calculating section 32 produces pulses having a large duty ratio so as to dilute the mixture. At a lean air-fuel mixture, the calculating section produces pulses having a small duty ratio so as to enrich the mixture. 
     A fixed duty ratio pulse generating section 38 is provided for providing various pulses in accordance with driving conditions. 
     The section 38 has a look-up table as shown in FIG. 3. The table is a three-dimensional table for producing a duty ratio signal dependent on an intake manifold vacuum signal by the output of the vacuum sensor 22 and on an engine speed signal which is obtained by ignition pulses. The table is made to provide various duty ratios which are proper for conditions of the engine during cold engine operation. The fixed duty ratio pulse generating section 38 is adapted to produce a plurality of pulse trains, each train having a fixed duty ratio which is determined by the look-up table in accordance with intake manifold vacuum (load on the engine) and engine speed. The fixed duty ratio pulses are applied to the electromagnetic valves 14 and 15 through the changeover circuit 39 and driver 36. 
     The changeover circuit 39 is operated by an output of a detecting circuit 40. The circuit 40 comprises an intake manifold vacuum detecting circuit 41 (comprising an acceleration detecting circuit), a coolant temperature detecting circuit 42 and an atmospheric pressure detecting circuit 43. The vacuum detecting circuit 41 is supplied with the output of the vacuum sensor 22 and produces a high level output when the vacuum (a value close to atmospheric pressure) is lower than a predetermined value (for example -300 mmHg), which means that the engine is greatly accelerated. 
     The coolant temperature detecting circuit 42 is applied with a signal from the coolant temperature sensor 20 and produces a high level output when the temperature is below 80° C. The atmospheric pressure detecting circuit 43 produces a high level output when the atmospheric pressure sensed by an atmospheric pressure sensor 23 is higher than 650 mmHg. The outputs of the circuits 41, 42 and 43 are applied to an AND gate 44, a high level output of which is applied to a timer 45 to operate it. The timer 45 produces a high level output for 0.2 seconds. Even if the high level output of the AND gate continues more than 0.2 seconds, the output of the timer 45 becomes low after 0.2 seconds. The high level output of the timer 45 operates the changeover circuit 39 to cut off the input from the calculating section 32 and to connect the output of the circuit 38 to the driver 36. 
     In cold engine operation at low altitude, when the manifold vacuum is higher than -300 mmHg. in a driving condition such as idling operation of the engine or steady state driving, the vacuum detecting circuit 41 produces a low level output, causing the output of AND gate 44 to go to a low level. Accordingly, the output of timer 45 is at a low level, so that the changeover circuit 39 connects the output of the calculating section 32 to the driver 36. Thus, the air-fuel ratio is controlled by the feedback control system. 
     When the manifold vacuum becomes lower than -300 mmHg by acceleration of the engine at cold engine operation at low altitude, the output of AND gate 44 goes to a high level, so that the output of timer 45 becomes high for 0.2 seconds at the most. Thus, during this period, pulses having duty ratios determined by the look-up table in accordance with the manifold vacuum and engine speed are applied from the section 38 to the electromagnetic valves 14 and 15 through the changeover circuit 39 and driver 36. Accordingly, the air-fuel ratio is controlled so as to meet the requirements at acceleration in cold engine operation. When one of inputs of AND gate 44 changes at a level, or after 0.2 seconds lapse, the output of the timer 45 goes to a low level. Thus, the system returns to the feedback control system. FIG. 4 shows the above described operation of the system. 
     While the presently preferred embodiment of the present invention has been shown and described, it is to be understood that this disclosure is for the purpose of illustration and that various changes and modifications may be made without departing from the scope of the invention as set forth in the appended claims.