Air-fuel ratio control system

A feedback air-fuel ratio control system for an internal combustion engine has an O.sub.2 -sensor for detecting the concentration of oxygen in exhaust gases of the engine, a coolant temperature sensor, and a circuit for disabling the feedback control operation at a low coolant temperature during warming-up of the engine. The feedback control system is also disabled when the engine is rapidly accelerated or decelerated, even if the engine is warming-up at coolant temperatures greater than the low coolant temperature state.

BACKGROUND OF THE INVENTION 
The present invention relates to an air-fuel ratio control system for an 
internal combustion engine for a motor vehicle provided with a three-way 
catalytic converter in an exhaust system, and with an O.sub.2 -sensor for 
detecting the oxygen concentration of exhaust gases, and more particularly 
to a feedback control system for controlling air-fuel ratio during rapid 
acceleration and deceleration of the motor vehicle. 
The air-fuel ratio control system responds to the feedback signal from the 
O.sub.2 -sensor to control the air-fuel ratio of the air-fuel mixture to a 
stoichiometric air-fuel ratio at which ratio the three-way converter is 
most effective. Such a feedback control operates under conditions of a 
coolant temperature higher than a predetermined value and when the 
activated O.sub.2 -sensor has a higher body temperature of the sensor than 
a predetermined value. When either the coolant temperature or O.sub.2 
-sensor temperature is lower than the predetermined values, the feedback 
control system is disabled and the air-fuel ratio control signal is fixed 
to a constant value to hold the air-fuel ratio at a fixed value. 
On the other hand, even if the feedback control operates under the normal 
conditions, when an accelerator pedal of the motor vehicle is fully 
depressed, the air-fuel ratio is fixed to enrich the mixture so as to meet 
the acceleration of the vehicle. Japanese Patent Laid Open No. 53-13021 
discloses an example of such an enrichment system. 
In order to increase the emission control range in engine operation, it is 
preferable to set the predetermined temperature, at which the feedback 
control system is enabled, to a lower value. On the contrary, in order to 
insure driveability during warming-up of the engine, it is preferable to 
set the temperature, at which the air-fuel ratio is fixed to enrich the 
mixture, to a higher value. To meet both requirements, usually the coolant 
temperature as the condition for enabling the feedback control system is 
set to a value between 20.degree. and 50.degree. C. 
However, intake air is not sufficiently pre-heated in the intake manifold 
of the engine at a coolant temperature below 80.degree. C., so that 
unvaporized fuel sticks to the inner wall of the intake manifold. The 
sticking of fuel to the wall causes the response of the feedback control 
operation to be delayed, increasing the amplitude of the oscillation of 
the system which increases deviation of the air-fuel ratio from the 
stoichiometric air-fuel ratio. Such a large deviation of the air-fuel 
ratio is further increased during rapid acceleration and rapid 
deceleration, causing an excessively rich or lean air-fuel mixture which 
causes the emission control effect to deteriorate. Japanese Patent Laid 
Open No. 56-162250 discloses an air-fuel ratio control system which 
operates to enrich the air-fuel mixture to a maximum rich value at a cold 
engine when rapidly accelerated. However, it is not always desirable to 
provide the maximum rich mixture. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide an air-fuel ratio control 
system which may properly control the air-fuel ratio at rapid acceleration 
and deceleration during cold engine operation to improve the emission 
control and the driveability of the vehicle. 
It is a further object of the present invention to provide a system 
comprising an O.sub.2 -sensor for detecting the concentration of oxygen in 
exhaust gases, an electronic feedback control circuit comprising a 
comparator for comparing an output signal of the O.sub.2 -sensor, control 
means responsive to an output of the comparator to produce a control 
signal, and a driving circuit responsive to the control signal for driving 
a fuel supply means and for controlling the air-fuel ratio to a value 
approximately equal to the stoichiometric air-fuel ratio. The system 
further comprises a coolant temperature sensor for producing a temperature 
signal dependent on the temperature of the coolant of the engine, first 
detecting means for detecting rapid acceleration and deceleration of the 
engine for producing an engine operation signal, and means operatively 
responsive to the temperature signal of the coolant temperature sensor, 
feedback control interrupting signal for the feedback control circuit when 
the coolant temperature is lower than a predetermined lower temperature or 
when the coolant temperature is lower than a predetermined higher 
temperature and the engine operative signal occurs. 
In an aspect of the present invention, the first detecting means is a 
throttle sensor for detecting the angular velocity of a throttle valve of 
the engine, and a second detecting means detects activation of the O.sub.2 
-sensor and comprises a deciding circuit for comparing the output voltage 
of the O.sub.2 -sensor with a reference voltage which is an O.sub.2 
-sensor activated voltage. When the reference voltage is not reached the 
feedback control interrupting signal is also produced. 
The other objects and features of this invention will be apparently 
understood from the following description with reference to the 
accompanying drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1 showing schematically an air-fuel ratio control system 
according to the present invention, an internal combustion engine 1 
mounted on a vehicle (not shown) is provided with an air flow meter 3 in 
an intake pipe 4 downstream of an air cleaner 2. The intake pipe 4 
communicates with an intake manifold 7 through a throttle body 6 having a 
throttle valve 5. The intake manifold 7 communicates with cylinders of the 
engine, which in turn communicates with an exhaust pipe 8 having a 
three-way catalytic converter 9. In the throttle body 6, a single 
fuel-injection valve 10 is provided. On the intake manifold 7, a water 
jacket 11 communicating with water jackets of the engine 1 is provided to 
pre-heat the intake air. 
A crankangle sensor 12 is provided for sensing engine speed (RPM), and an 
O.sub.2 -sensor 13 is provided on the exhaust pipe 8 upstream of the 
converter 9 to sense the concentration of oxygen in the exhaust gases. 
Further, a coolant temperature sensor 14 and a throttle sensor 15 for 
detecting the angular velocity of the throttle valve 5 are provided. 
Outputs of these sensors 3, 12-15 are applied to a control unit 16 to 
operate the fuel-injection valve 10 so as to inject the fuel in accordance 
with an injection signal. 
Referring to FIGS. 2a, 2b outputs of the air flow meter 3 and crankangle 
sensor 12 are applied to an intake air quantity computing circuit 17 and 
to an engine speed computing circuit 18, respectively. Intake air quantity 
Q and engine speed N are applied to a base injection-pulse width computing 
circuit 19 which produces a base injection-pulse width signal dependent on 
the signals Q and N. The base injection-pulse width signal is applied to 
an injection-pulse width computing circuit 20. The circuit 20 operates to 
correct the base injection-pulse width signal in accordance with signals 
from the coolant temperature sensor 14 and the throttle sensor 15 to 
produce an injection-pulse width signal dependent on cold engine operation 
and acceleration or deceleration of the engine. The injection-pulse width 
signal is fed to the fuel injector 10 through a driver 21 to inject the 
fuel. 
The air-fuel ratio feedback control system comprises an air-fuel ratio 
deciding circuit 22 applied with the output of the O.sub.2 -sensor 13, a 
control signal deciding circuit 23 applied with the output of the air-fuel 
ratio deciding circuit 22, and a correcting circuit 24 which produces a 
correcting quantity signal applied to the injection-pulse width computing 
circuit 20. 
The output signal of the coolant temperature sensor 14, representing the 
coolant temperature Tw, is applied to a comparing circuit 25, where the 
output signal is compared with a feedback control executing temperature 
Tw.sub.1 (20.degree.-50.degree. C.) and with a feedback control 
interrupting temperature Tw.sub.2 (50.degree.-80.degree. C.) at rapid 
acceleration and deceleration. The output signal of the comparing circuit 
25 is applied to a coolant temperature deciding circuit 26. On the other 
hand, the output of the O.sub.2 -sensor 13 is applied to an activation 
deciding circuit 27 in which the output voltage Vo.sub.1 of the O.sub.2 
-sensor is compared with a reference voltage Vo.sub.2 for deciding whether 
the O.sub.2 -sensor 13 is activated. The output of the throttle sensor 15 
is applied to a computing circuit 28 to compute the angular velocity 
.alpha. of the throttle value. The output of the computing circuit 28 
representing the angular velocity .alpha. is applied to a rapid 
acceleration and deceleration deciding circuit 29. Output signals of the 
circuits 26, 27 and 29 are applied to a feedback control deciding circuit 
30, the output of which is applied to the control signal deciding circuit 
23. 
Explaining the operation of the system with reference to FIGS. 2a, 2b and 
3, the injection-pulse width computing circuit 20 computes the quantity of 
fuel injected to one cylinder from outputs of the air flow-meter 3, 
crankangle sensor 12, O.sub.2 -sensor 13, coolant temperature sensor 14, 
and throttle sensor 15. When the coolant temperature Tw is lower than the 
feedback control executing temperature Tw.sub.1, or the output voltage 
Vo.sub.1 of the O.sub.2 -sensor 13 does not reach the O.sub.2 -sensor 
activated voltage Vo.sub.2, or the control temperature TW is lower than 
the feedback control interrupting temperature TW.sub.2 when rapid 
acceleration or deceleration occurs, the feedback control deciding circuit 
30 produces a feedback control interrupt signal which is applied to the 
control signal deciding circuit 23. In accordance with the interrupt 
signal, the circuit 23 produces a control signal for interrupting the 
feedback control. Accordingly, the injection-pulse width computing circuit 
20 produces an output signal, so that the fuel injector 10 injects the 
fuel regardless of the output of the O.sub.2 -sensor 13. 
When the coolant temperature Tw is higher than the temperature Tw.sub.1, 
and the voltage Vo.sub.1 is higher than the voltage Vo.sub.2, and the 
coolant temperature Tw is higher than the temperature Tw.sub.2 or if it is 
lower than the temperature Tw.sub.2 when rapid acceleration or 
deceleration does not occur, the feedback control deciding circuit 30 
produces a feedback control executing signal which is applied to the 
control signal deciding circuit 23. Thus, the injection-pulse width 
computing circuit 20 produces an output signal corrected by the output 
signal of the O.sub.2 -sensor. When the air-fuel ratio is larger than the 
stoichiometry (lean air-fuel mixture), the circuit 20 produces an 
enriching signal, and vice versa. Thus, the feedback control operation is 
performed, although the coolant temperature Tw is lower than the 
temperature Tw.sub.2 when rapid acceleration or deceleration does not 
occur. 
Under the feedback control operation at a coolant temperature lower than 
the feedback control interrupt temperature Tw.sub.2, when the occurrence 
of rapid acceleration or deceleration is determined by the circuit 29, the 
feedback control deciding circuit 30 produces the feedback control 
interrupt signal. Thus, the fuel injector 10 injects the fuel in 
accordance with the signal. 
When the coolant temperature Tw is higher than the temperature Tw.sub.2, 
which means the completion of warming-up of the engine, the feedback 
control is executed. Since the fuel is sufficiently vaporized by the 
intake air pre-heated by the coolant in the water jacket 11, the feedback 
control operation can be performed without delay. 
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 spirit and scope of the invention as 
set forth in the appended claims.