Air-fuel ratio control system

An air-fuel ratio control system for an internal combustion engine has an electromagnetic valve for correcting the air-fuel ratio of air-fuel mixture, and an O.sub.2 sensor for detecting oxygen concentration in exhaust gases. A detecting circuit is provided for producing an acceleration signal when the engine is accelerated at cold engine operation. A pulse generating circuit is provided for producing a plurality of pulses, the duty ratios of which are proper for air-fuel ratio at acceleration in cold engine operation. A changeover switch is provided to respond to the acceleration signal for applying the pulses to the electromagnetic valve so as to control the air-fuel ratio.

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.sub.2 
sensor provided in the exhaust system of the engine, and a decision is 
made dependent on the output signal from the O.sub.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.

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.sub.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.sub.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.sub.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.sub.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.degree. 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.