System for controlling air-fuel ratio

A system for controlling the air-fuel ratio for an internal combustion engine having an intake passage, an exhaust passage, a detector such as an oxygen sensor for detecting the concentration of oxygen in the exhaust gases, an air-fuel mixture supply unit, an on-off type electromagnetic valve for correcting the air-fuel ratio of the air-fuel mixture supplied by the air-fuel mixture supply unit, and an electronic control circuit for producing square wave pulses in dependency on the output signal from the detector for driving the on-off type electromagnetic valve. The system comprises a venturi for producing a vacuum dependent on rapid acceleration, a vacuum sensor for sensing the vacuum in the venturi at rapid acceleration of the internal combustion engine and producing an output signal dependent thereon, an intake opening in the venturi for communicating the vacuum with said vacuum sensor, pulse width modulator for producing a pulse width modulating signal when an output signal of the vacuum sensor rises above a predetermined level at a rapid acceleration, the pulse width modulator being connected to the electronic control circuit, such that said pulse width modulating signal is fed to said electronic control circuit for modulating the width of said square wave pulses in dependency on the rapid acceleration for enriching the air-fuel mixture.

BACKGROUND OF THE INVENTION 
The present invention relates to a system and method for controlling the 
air-fuel ratio for an internal combustion engine emission control system 
with a three-way catalyst, and more particularly to a system for 
correcting the deviation of the air-fuel ratio during rapid acceleration 
of the engine. 
Such a control system is, as in U.S. Pat. No. 4,132,199, a feedback control 
system, in which an oxygen sensor is provided to sense the oxygen 
concentration in exhaust gases to generate an electrical signal as an 
indication of the air-fuel ratio of the burned air-fuel mixture. The 
control system operates to actuate an air-fuel mixture supply means to 
control the air-fuel ratio of the mixture to the stoichiometric air-fuel 
ratio according to the signal from the oxygen sensor. 
The system may sufficiently control the air-fuel ratio during the usual 
operation of the engine. However, during rapid acceleration of the engine, 
the system cannot immediately respond to the variation of the air-fuel 
ratio of the mixture as described hereinafter. 
If the engine is rapidly accelerated, the amount of induced air immediately 
increases by an increase of the vacuum in the intake passage. On the other 
hand, the air-fuel mixture supply means does not rapidly operate in 
response to the increase of the amount of induced air. As a result, the 
air-fuel ratio increases and consequently, a lean air-fuel mixture is 
supplied. The air-fuel ratio gradually decreases to a proper ratio as the 
speed of the engine increases. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a system which can correct 
the deviation of the air-fuel ratio to the lean side just after the rapid 
acceleration of the engine. 
According to the present invention, there is provided a system for 
controlling the air-fuel ratio for an internal combustion engine having an 
intake passage, an exhaust passage, a throttle valve, detecting means for 
detecting the concentration of a constituent of the exhaust gases passing 
through the exhaust passage, air-fuel mixture supply means, an electronic 
control circuit for producing square wave pulses in accordance with the 
output signal of said detecting means and an on-off type electromagnetic 
valve actuated by the square wave pulses from the electronic control 
circuit for correcting the air-fuel ratio of the air-fuel mixture supplied 
by said air-fuel mixture supply means, the system comprising a venturi for 
producing a vacuum dependent on rapid acceleration, a vacuum sensor for 
sensing the vacuum in the venturi at rapid acceleration of the internal 
combustion engine and producing an output signal dependent thereon, an 
intake opening in the venturi for communicating the vacuum with said 
vacuum sensor, pulse width modulator for producing a pulse width 
modulating signal when the output signal of the vacuum sensor rises above 
a predetermined level at a rapid acceleration, the pulse width modulator 
being connected to the electronic control circuit, such that said pulse 
width modulating signal is fed to said electronic control circuit for 
modulating the width of said square wave pulses in dependency on the rapid 
acceleration for enriching the air-fuel mixture. 
Other objects and features of the present invention will become apparent 
from the following description of a preferred embodiment with reference to 
the accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, a carburetor 1 communicates with an internal 
combustion engine (not shown). The carburetor 1 comprises a float chamber 
2, a venturi 3 formed in an intake passage 1a, a nozzle 4 communicating 
with the float chamber 2 through a main fuel passage 5, and a slow port 9 
provided near a throttle valve 8 in the intake passage communicating with 
the float chamber 2 through a slow fuel passage 10. Air correcting 
passages 7 and 12 are disposed in parallel to a main air bleed 6 and a 
slow air bleed 11, respectively. On-off type electromagnetic valves 13 and 
14 are provided for the air correcting passages 7 and 12, respectively. 
Inlet ports 13a and 14a of each on-off electromagnetic valve 13 and 14 
respectively communicates with the atmosphere through an air filter or air 
cleaner 15. An oxygen sensor 17 is disposed in an exhaust pipe 16 which 
communicates with the internal combustion engine. The sensor 17 detects 
the oxygen content of the exhaust gases. A three-way catalytic converter 
(not shown) is provided in the exhaust pipe 16 downstream of the oxygen 
sensor 17. A vacuum sensor 19 communicates with the venturi 3 and is 
responsive to the vacuum condition therein. The sensor 19 comprises a 
potentiometer (known per se) operatively communicating with the venturi 3 
for converting the vacuum in the intake passage into a voltage signal. The 
output signal of the oxygen sensor 17 is applied to a comparing circuit 20 
of an electronic control system. The comparing circuit 20 operates to 
compare the input signal from the oxygen sensor 17 with a reference value 
V.sub.R corresponding to the stoichiometric air-fuel ratio and to 
determine whether the input signal is rich or lean compared with the 
reference stoichiometric air-fuel ratio to produce a comparison signal 
dependent thereon. 
The comparison signal is applied to an integration circuit 21, where the 
signal is converted into an integration signal which varies in an opposite 
direction to the direction represented by the input comparison signal. The 
integration signal is compared in a comparator 22 with triangular wave 
pulses applied from a standard triangular wave pulse generator 23 so that 
square wave pulses are produced at the output of the comparator 22. The 
square wave pulses are fed to both of the on-off type electromagnetic 
valves 13 and 14 through a driving circuit 24. 
When a rich air-fuel ratio is determined, the comparator 22 produces output 
pulses having a greater pulse duty ratio, whereby the opening times of the 
on-off type electromagnetic valves 13 and 14 increase and as a result the 
amount of air passing through the valves 13 and 14 increases. Thus, the 
amount of air in the mixture fed from the carburetor 1 increases to 
thereby increase the air-fuel ratio. When a lean air-fuel ratio is 
determined, the output of the comparator 22 has a smaller pulse duty 
ratio, whereby the air-fuel ratio is decreased to enrich the mixture fed 
from the carburetor. 
In accordance with the present invention, the voltage output of the vacuum 
sensor 19 is connected to a pulse width modulating circuit 25. The pulse 
width modulating circuit 25 is designed so as to produce pulse width 
modulating signals D, E (FIG. 5) when the signal from the vacuum sensor 19 
exceeds a predetermined level by a rapid acceleration. The amount of the 
pulse width modulating signal increases with increasing vacuum pressure as 
shown in FIG. 2. 
When the throttle valve 8 is rapidly opened for acceleration and the vacuum 
pressure rises above a predetermined level as shown in FIGS. 3(a), (b), 
the pulse width modulating circuit operates to produce pulse width 
modulating signals D, E (FIG. 5). The pulse width modulating signals D, E 
(FIG. 5) are fed to the integration circuit 21 and the corrected output 
signal thereof is to fed to the input of the comparator 22. Thus, the 
output signal from the integration circuit 21 is corrected, and the pulse 
width of the pulse produced by the comparator circuit 22 is changed. FIG. 
3(c) shows such a pulse width as indicated by dashed lines. By the pulse 
width modulation, the pulse duty ratio of the electromagnetic valves 13 
and 14 is decreased, so that the air-fuel ratio of the mixture is 
decreased. Thus, the mixture can be enriched in accordance with the 
increase of the vacuum pressure in the acceleration. The enrichment is 
effected during rapid acceleration only. Therefore, an excessive 
enrichment of the mixture during the usual operation of the engine can be 
prevented. 
FIG. 4 shows an electronic control circuit, in which each block depicted by 
dash dotted lines corresponds to that of the block diagram of FIG. 1. The 
pulse width modulating circuit 25 of FIG. 4 comprises a one pulse 
generating circuit 26, a small width pulse generating circuit 27, and a 
fixed duty ratio voltage source 29. When the output voltage of the vacuum 
sensor 19 exceeds the predetermined level, an operational amplifier 30 in 
the circuit 26 produces an output signal (C) (FIG. 5(c)) and the circuit 
26 generates a pulse (E) (FIG. 5(E)). By the pulse (E), the circuit 27 
generates a small pulse (D) which is fed to gates of switches 31 and 32 to 
close them and to the gate of a switch 33 via an inverter 34 to open it. 
The switches 31 and 32 are closed and the switch 33 is open during this 
pulse (D). Further, the pulse (E) is fed to the gate of a switch 35 and to 
the gate of a switch 36 through a transistor 37. Consequently, the 
capacitor in an integrator 38 in the integration circuit 21 is discharged 
during the pulse D and the gain of the feedback circuit 21 is increased 
during the pulse E. But during the pulse (D) closing the switch 32 and 
opening switch 31, the fixed duty ratio voltage for enrichment of the 
mixture is supplied from the fixed duty ratio voltage source 29 to the 
comparator 22 causing the output signal A to drop to a predetermined fixed 
low value f. After the short pulse D ends switches 31 and 32 open and 
switch 33 closes enabling the increased gain output (effected by 
continuing signal E) of circuit 21 to provide corrected slope g at signal 
A. FIG. 5(B) shows the output of the oxygen sensor 17 at (B) in FIG. 4 and 
FIG. 5(A) shows the control signal at (A) which is corrected by the pulse 
width modulating signals. The corrected control signal f, g causes the 
comparator 22 to produce a small duty ratio pulse signal. Thus, the on-off 
type electromagnetic valves 13, 14 are closed longer and less air enters 
the air-fuel mixture, whereby the mixture supplied by the carburetor is 
enriched during the pulse (E). 
In accordance with the present invention, a temporary deviation of the 
air-fuel ratio toward the lean side during rapid acceleration can be 
prevented, whereby the acceleration performance of the engine can be 
improved.