Air-to-fuel ratio control system for an engine

In a carburetor in which air-to-fuel ratio is feedback controlled into a stoichiometric mixture ratio in response to oxygen sensor signals, air-to-fuel ratio is determined to a lower value to supply a rich mixture to an engine when intake air temperature and engine coolant temperature both exceed each reference value, so that engine overheat can be prevented at high temperatures. The lower air-to-fuel ratio is determined to a fixed value or to an appropriate value under consideration of intake air temperature and engine coolant temperature or additionally vehicle speed.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates generally to an air-to-fuel ratio control 
system for an engine and more specifically to an air-to-fuel ratio control 
system incorporated with an electronically controlled carburetor for 
prevention of engine overheat. 
2. Description of the Prior Art 
Air-to-fuel ratio control systems used with an electronically-controlled 
carburetor are well known. An example of these systems is disclosed in 
Japan Published Unexamined Patent Application No. 52-129841, entitled 
Air-to-Fuel Ratio Control System on Engine Closed Loop. In this 
application, air-to-fuel ratio is controlled to a stoichiometric mixture 
ratio by actuating an electromagnetic valve provided for a carburetor 
connected to an engine intake passage, in order to increase or decrease 
the amount of fuel supplied to the engine. The electromagnetic valve is 
feedback controlled in response to signals outputted from an oxygen sensor 
for detecting oxygen concentration in engine exhaust gas. Further, intake 
air vacuum is detected for correcting air-to-fuel ratio to a rich mixture 
when engine load is heavy, thus improving engine operating characteristics 
under heavy engine load. 
In the prior-art air-to-fuel ratio control system as described above, 
however, although engine operating characteristics are improved under a 
heavy engine load, no consideration is taken for engine operating 
characteristics of when intake air temperature or the engine coolant 
temperature is high. 
Therefore, even at high temperatures, air-to-fuel ratio is controlled to a 
stoichiometric mixture value, so that combustion temperature is high in 
engine combustion chamber. This results in a problem in that exhaust gas 
temperature rises and additionally the engine is easily overheated. 
A more detailed description of the prior-art air-to-fuel ratio control 
system will be described in more detail under DETAILED DESCRIPTION OF THE 
PREFERRED EMBODIMENTS. 
SUMMARY OF THE INVENTION 
With these problems in mind, therefore, it is the primary object of the 
present invention to provide an air-to-fuel ratio control system 
incorporated with a carburetor which can prevent engine overheat when 
intake air temperature and engine coolant temperature are both high. 
To achieve the above-mentioned object, the air-to-fuel ratio control system 
for an engine according to the present invention comprises (a) an oxygen 
sensor for detecting oxygen concentration in engine exhaust gas and for 
outputting an oxygen sensor signal; (b) an engine coolant temperature 
sensor for outputting an engine coolant temperature signal; (c) an intake 
air temperature sensor for outputting an intake air temperature signal; 
(d) air-to-fuel ratio control means (103) for correcting the amount of 
fuel to be supplied to the engine so as to obtain mixture with a 
stoichiometric mixture ratio in response to the detected oxygen sensor 
signal in acordance with feedback method when the detected engine coolant 
temperature of the detected intake air temperature is below each reference 
value, for increasing the amount of fuel to be supplied to the engine so 
as to obtain rich mixture irrespective of the oxygen sensor signal for 
prevention of engine overheat when the detected engine coolant temperature 
and the detected intake air temperature exceed each reference value, and 
for outputting a control signal representative of the amount of fuel; and 
(e) an actuator associated with the carburetor and activated in response 
to the control signal outputted from said control means. 
To achieve the above-mentioned object, the method of increasing fuel 
supplied to an engine for prevention of engine overheat according to the 
present inention comprises the following steps of (a) detecting intake air 
temperature T.sub.A ; (b) comparing the detected intake air temperature 
T.sub.A with a reference value T.sub.AO ; (c) if the detected intake air 
temperature T.sub.A is lower than the reference value T.sub.AO, supplying 
fuel into the engine in response to the oxygen sensor signals and in 
accordance with feedback control method; (d) if the detected intake air 
temperature T.sub.A is higher than the reference value T.sub.AO, detecting 
engine coolant temperature T.sub.C ; (e) comparing the detected engine 
coolant temperature T.sub.C with a reference value T.sub.CO ; (f) if the 
detected engine coolant temperature T.sub.C is lower than the reference 
value T.sub.CO, supplying fuel into the engine in response to oxygen 
sensor signals and in accordance with feedback control method; (g) if the 
detected engine coolant temperature T.sub.C is higher than the reference 
value T.sub.CO, selecting a duty factor from a look-up table under 
consideration of the detected intake air temperature T.sub.A and the 
detected engine coolant temperature T.sub.C ; (h) generating a control 
pulse signal having the selected duty factor; and (i) activating an 
actuator associated with the carburetor in response to the control pulse 
signal to increase the amount of fuel to be supplied to the engine through 
the carburetor.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
To facilitate understanding of the present invention, a brief reference is 
made to an example of prior-art air-to-fuel ratio control systems for an 
engine disclosed in Japan Published Unexamined Patent Application No. 
52-129841, entitled Air-to-Fuel Ratio Control System on Engine Closed 
Loop, with reference to the attached drawings. 
In FIG. 1A, a carburetor 1 includes a metering jet 2, a power jet 3 and an 
air bleeder 4. The power jet 3 is adjustably opened by a power mechanism 5 
actuated by a vacuum developed on the downstream side of a throttle valve 
6. The air bleeder 4 is also adjustably opened by an electromagnetic valve 
7 to control mixture to an appropriate air-to-fuel ratio. The 
electromagnetic valve 7 is controlled by a control unit 10 as shown in 
FIG. 1(B). 
The control unit 10 is made up of a reference voltage generator 11, a 
triangular wave signal generator 12, two comparators 13 and 14, an oxygen 
sensor 15, a difference-to-time converter 16, an integrating circuit with 
a resistor 17 and a capacitor 18, a power transistor 19, a diode 20, a 
normally-open contact 21 and a vacuum switch 22. 
The oxygen sensor 15 generates an electromotive force according to the 
ratio of oxygen concentration in atmosphere to that in exhaust gas. In 
other words, a positive voltage is generated when air-to-fuel ratio is 
below a stoichiometric mixture ratio (rich mixture or insufficient oxygen) 
and no voltage is generated when air-to-fuel ratio is beyond the 
stoichiometric mixture ratio (lean mixture or excessive oxygen). The 
difference-to-time converter 16 converters a difference in voltage between 
a reference triangular wave signal and oxygen sensor signal into a time 
interval signal corresponding thereto. That is to say, when the oxygen 
sensor 15 outputs a high voltage level signal (rich mixture), the 
converter 16 outputs a signal to activate the electromagnetic valve 7 for 
a converted time period in order to open the air bleeder 4 or to control 
the mixture into a lean mixture; when th oxygen sensor 15 outputs a low 
voltage level signal (lean mixture), the converter 16 outputs no signal to 
deactivate the electromagnetic valve 7 in order to close the air bleeder 
4. 
The vacuum switch 22 can close the contact 21 when intake vacuum drops 
below a predetermined value (e.g. -200 mHg) under a heavy engine load. 
When the contact 21 is closed, since the emitter and the collector of the 
power transistor 19 is shorted, the electromagnetic valve 7 is held at a 
constant voltage level determined by a resistor 23, irrespective of the 
signal level outputted from the integrating circuit 17 and 18 or the 
oxygen sensor 15. 
In the above-mentioned air-to-fuel ratio control system, air-to-fuel ratio 
is feedback controlled to a stoichiometric mixture ratio by detecting 
oxygen concentration within the exhaust gas. Further, when engine load is 
heavy, and therefore intake vacuum is reduced, the control unit 10 holds 
the electromagnetic valve 7, irrespective of the signal from the oxygen 
sensor 15 so that the electromagnetic valve 7 is kept closed to keep 
carburetor air-to-fuel ratio a little richer than the stoichiometric 
mixture ratio. Thereafter, when the throttle valve is opened and therefore 
intake vacuum is increased, the power jet 3 is opened by the vacuum to 
further obtain a rich mixture. In summary, in this system, the air-to-fuel 
ratio is feedback controlled by the oxygen sensor 15 for maximizing the 
efficiency of exhaust gas purification, but the feedback loop is held at a 
constant level under heavy engine load for maximizing the efficiency of 
engine power. 
In the prior-art air-to-fuel ratio control system as described above, 
although engine operating characteristics are improved under a heavy 
engine load, since no special countermeasures are considered against high 
intake air temperature or high engine coolant temperature, the engine will 
easily be overheated at high air temperatures or high coolant 
temperatures. 
In view of the above description, reference is now made to a basic 
embodiment of the air-to-fuel ratio control system for an engine according 
to the present invention with reference to FIG. 2. The system comprises an 
oxygen sensor 15, an engine coolant temperataure sensor 101, an intake air 
temperataure sensor 102, an air-to-fuel ratio control means 103, an 
actuator 104 and a carburetor 1. 
The oxygen sensor 15 outputs a high-voltage level signal when mixture is 
rich (air-to-fuel ratio is lower than a stoichiometric mixture ratio) but 
a low-voltage level signal when mixture is lean (air-to-fuel ratio is 
higher than the stoichiometric mixture ratio), being disposed within an 
exhaust pipe of an engine. 
The engine coolant temperature sensor 101 detects the temperature T.sub.C 
of engine coolant; the intake air temperature sensor 102 detects the 
temperature T.sub.A of intake air introduced into the engine. 
The air-to-fuel ratio control means 103 determines the amount of fuel to be 
supplied to the engine in response to signals outputted from the oxygen 
sensor 15 so that air-to-fuel ratio reaches a target value or a 
stoichiometric mixture ratio in accordance with feedback method, when the 
detected engine coolant temperature or the detected intake air temperature 
is below each reference value; and further determines the amount of fuel 
to be supplied to the engine, irrespective of the oxygen sensor signal, so 
that air-to-fuel ratio reaches a richer value in accordance with table 
look-up method of prevention of engine overheat, when the detected engine 
coolant temperature and the detected air temperature exceed each reference 
value. The control means 103 outputs a control signal representative of 
the determined amount of fuel to be supplied. 
The actuator 104 is activated in response to the control signal from the 
air-to-fuel ratio control means 103. the carburetor 1 supplies an 
appropriate amount of fuel to the engine according to the amount of intake 
air and increases or decreases the amount of fuel to be supplied in 
response to the control signal outputted from the control means 103. In 
the air-to-fuel ratio control system as described above, in particular, a 
rich mixture is supplied from the carburetor to the engine when engine 
coolant temperature T.sub.C and intake air temperature T.sub.A exceed the 
respective reference values, simultaneously, in order to reduce combustion 
temperature or to prevent engine overheat at high temperatures. In FIG. 2, 
the reference numeral 105 denotes a transmission gear shift lever position 
sensor which can outputs a signal when a gear shift lever is set to Park 
or Neutral other than Drive positions. The reference numeral 106 denotes a 
vehicle speed sensor which can output a signal when vehicle speed is zero. 
Since engine is readily overheated when vehicle is at rest, it is 
preferable to take vehicle speed into consideration when supplying a rich 
mixture for prevention of overheat. 
FIG. 3 shows an embodiment of the air-to-fuel ratio control system 
according to the present invention. An engine 51 is provided with a 
combustion chamber 52, into which mixture is supplied through an intake 
pipe 54. The mixture is obtained by mixing intake air cleaned through an 
air cleaner 53 with fuel supplied through a carburetor 1. Exhaust gas 
obtained after the mixture has been burnt out within the combustion 
chamber 52 is introduced through an exhaust pipe 55 to a ternary catalyst 
converter 56 and then exhausted out. The catalyst converter 56 purifies 
the exhaust gas by oxidizing chemical components of HC and CO and by 
deoxidizing chemical component NOx all included in exhaust gas. 
The intake air temperature sensor 102 for detecting intake air temperature 
T.sub.A is disposed at an appropriate position within the air cleaner 53. 
However, it is also possible to dispose this intake air temperature sensor 
102 within the intake pipe 54 for detection of mixture temperature on the 
downstream side of throttle valves 6A and 6B. Further, it is also possible 
to dispose this intake air temperature sensor 102 on the outside of the 
air cleaner 53 for detection of outside air temperature. 
The engine coolant temperature sensor 101 for detecting engine coolant 
temperature T.sub.C is disposed at an appropriate position of an engine 
cylinder block 57 or a radiator (not shown). 
The oxygen sensor 15 for detecting oxygen concentration O.sub.2 in exhaust 
gas is disposed at an appropriate position of an exhaust pipe 55. The 
oxygen sensor 15 outputs a high-voltage level signal when air-to-fuel 
ratio is below a stoichiometric mixture ratio or a rich mixture is 
supplied and therefor oxygen is insufficient but a low-voltage level 
signal when air-to-fuel ratio is beyond the stoichiometric mixture ratio 
or a lean mixture is supplied and therefore oxygen is excessive. 
The electronically-controlled carburetor 1 is formed with a primary passage 
1A within which a primary throttle valve 6A is disposed and a secondary 
passage 1B within which a secondary throttle valve 6B is disposed. 
Further, the carburetor 1 is formed with a first passage 1C communicating 
with the upsteam side of a primary venturi portion, a second passage 1D 
communicating with the downstream side of a primary throttle valve 6A, and 
a third passage 1E communicating with the primary venturi portion. In FIG. 
3, the reference numeral 1F denotes a solenoid valve and the reference 
numeral 1G denotes a float chamber. A main jet 1H is formed between the 
solenoid valve 1F and the float chamber 1G on the low side and an 
auxiliary jet 1I is formed between the solenoid valve 1F and the float 
chamber 1G on the upper side both for supplying fuel within the float 
chamber 1G to the primary venturi portion. 
In the above-mentioned carburetor 1, when the solenoid valve 1F is 
energized, two upper and lower valve members are moved in the upward 
direction, so that the upper valve member opens to communicate the first 
and second passages 1C and 1D with the third passage 1E and the lower 
valve member opens the main jet 1H but closes the auxiliary jet 1I. 
Therefore, a vacuum applied to the main jet 1H is reduced and further fuel 
is supplied from only the main jet 1H to the venturi portion. Under these 
conditions, the amount of fuel to be supplied is relatively small. 
In contrast with this, when the solenoid valve 1F is deenergized, two upper 
and lower valve members are moved in the downward direction, so that the 
upper valve member closes not to communicate the first and second passages 
1C and 1D with the third passage 1E and the lower valve member opens both 
the main jet 1H and the auxiliary jet 1I. Therefore, vacuum applied to the 
main jet 1H is increased and further fuel is supplied from both the main 
jet 1H and the auxiliary jet 1I to the venturi portion. Under these 
conditions, the amount of fuel to be supplied is relatively great. 
The solenoid valve 1F is controlled in response to a control pulse signal 
D, the duty factor of which is determined by the air-to-fuel ratio control 
means 103. Duty factor is a ratio (tw/T) of pulse width (Tw) to pulse 
period (T). Therefore, the more the duty factor D, the more the solenoid 
valve 1F will be energized to reduce the amount of fuel to be supplied; 
the less the duty factor, the less the solenoid valve 1F wil be energized 
to increase the amount of fuel to be supplied. However, it is of course 
possible to reverse the relationship between the duty factor and the 
amount of fuel by changing the direction that the solenoid valve 1F is 
driven when energized. 
Further, although not shown, another secondary main jet is provided so as 
to supply fuel to the venturi portion of the secondary passage 15. In 
summary, the carburetor 1 supplies fuel to the engine 51 corresponding to 
the amount of intake air through the primary and secondary main jets and 
further increases or decreases fuel to be supplied to the engine through 
the auxiliary jet 1I in response to the control pulse signal D with 
variable duty factor outputted from the air-to-fuel ratio control means 
103. 
The air-to-fuel control means 103 is a microcomputer made up of a central 
processing unit (CPU) 103A, memory units 103B including read-only memory 
(ROM) and random access memory (RAM) and an input/output port including 
analog-to-digital converters and digital-to-analog converters. The 
detection signals outputted from the three sensors 15, 101 and 102 (oxygen 
concentration O.sub.2, air temperature T.sub.A and coolant temperature 
T.sub.C) are all inputted to the control means 103 through the I/O port 
103C, through which analog signals are converted digital signals 
corresponding thereto where necessary. The CPU 105A reads 
externally-detected data signals, transfers or receives the read data 
signals to and from the RAM for executing data processing in accordance 
with control program stored in the ROM and outputs a control signal 
through the I/O port 103C. Further, in the memory unit 103B, necessary 
data are previously stored in the form of tables as described later in 
more detail. 
The operation of the air-to-fuel ratio control system according to the 
present invention will be described hereinbelow. 
In the carburetor 1, fuel is supplied from the float chamber 1G to the 
primary and secondary venturi portions arranged within the primary and 
secondary passages 1A and 1B through the primary and secondary main jets. 
Since the vacuum is increased in proportion to an increase in the amount 
of intake air, the fuel supplied through the two venturi portions is 
roughly proportional to the amount of intake air. In addition to the above 
mentioned fuel supply through the primary and secondary main jets, fuel 
supplied to the primary venturi portion 1A is increased or decreased 
through the auxiliary jet 1I in response to the control pulse signal D 
outputted from the control means 103 to the solenoid valve 1F. In more 
detail, the control means 103 first determines the amount of fuel to be 
adjusted through the auxiliary jet 1I on the basis of engine coolant 
temperature T.sub.C detected by the coolant sensor 101 and then corrects 
the amount of fuel on the basis of the output signal from the oxygen 
sensor 15. That is to say, when coolant temperature T.sub.C is lower, the 
control means 103 decreases the duty factor of the control signal D to 
increase fuel to be supplied through the auxiliary jet 1I; when coolant 
temperature T.sub.C is sufficiently high, the control means 103 increases 
the duty factor of the control signal D to decrease fuel to be supplied 
through the auxiliary jet 1I. Additionally, when the oxygen sensor 15 
outputs a high-voltage level signal indicative of rich mixture, the 
control means 103 increases the duty factor of the control signal D to 
decrease fuel to be supplied through the auxiliary jet 1I; when the oxygen 
sensor 15 outputs a low-voltage level signal indicative of lean mixture, 
the control means 103 decreases the duty factor of the control signal D to 
increase fuel to be supplied through the auxiliary jet 1I. In summary, 
air-to-fuel ratio is feedback controlled in response to the oxygen 
concentration signal outputted from the oxygen sensor 15. Further, in this 
feedback control method, the amount of fuel to be corrected is adjusted in 
accordance with proportional-plus-integral control action (PI control), in 
which fuel is corrected in proportion to an addition of the error signal 
(H-level signal) and its integral. 
In addition to the above-mentioned feedback control, the control means 103 
fixedly determines the air-to-fuel ratio at predetermined values (rich 
mixture), when intake air temperature T.sub.A and engine coolant 
temperature T.sub.C both exceed respective reference values, irrespective 
of the detection signal from the oxygen sensor 15. To fix the air-to-fuel 
ratio, the control means 103 determines a duty factor (e.g. 10 percent) of 
the control pulse signal D applied to the solenoid valve 1F in accordance 
with table look-up method. In this table, various duty factors are listed 
with intake air temperature T.sub.A and coolant temperature T.sub.C as 
parameters. 
These fixed duty factor Do may be determined under consideration of 
transmission gear shift lever position or vehicle speed in addition to the 
intake air T.sub.A or coolant temperature T.sub.C. This is because when 
the transmission is shifted to Part or Neutral or when vehicle speed is 
zero, engine may easily be overheated. Furthermore, it is also possible to 
determine only a single duty factor without changing it according to 
various parameters. Further, it should be noted that when temperatures 
T.sub.A and T.sub.C exceed both each predetermined value, the duty factor 
is so determined as to obtain a rich mixture (low air-to-fuel ratio). This 
is because when mixture is rich, since oxygen becomes insufficient, fuel 
is burnt imperfectly and therefore combustion temperature is reduced, thus 
it being possible to prevent engine overheat. 
The above-mentioned method of determining a rich mixture at high 
temperatures T.sub.A and T.sub.C will be described in detail with 
reference to a control flowchart shown in FIG. 4, in which S.sub.1 to 
S.sub.7 denote each control step. 
The control means 103 first reads an intake air temperature T.sub.A from 
the intake air temperature sensor 102. in step S.sub.1. In step S.sub.2, 
control compares the read intake air temperature T.sub.A with a reference 
temperature T.sub.AO (e.g. 65.degree. C.). If T.sub.A is lower than 
T.sub.AO, program control advance to step S.sub.7 to feedback control the 
air-to-fuel ratio in response to the detected oxygen sensor signal. In 
step S.sub.2, if T.sub.A is higher than T.sub.AO, program control further 
reads an engine coolant temperature T.sub.C from the coolant temperature 
sensor 101 in step S.sub.3. In step S.sub.4, control compares the read 
coolant temperature T.sub.C with a reference temperature T.sub.CO (e.g. 
105.degree. C.). If T.sub.C is lower than T.sub.CO, program control 
advances to the step S.sub.7 to similarly feedback control the air-to-fuel 
ratio in response to the detected oxygen sensor signal. In step S.sub.4, 
if T.sub.A is higher than T.sub.CO, program control advances to step 
S.sub.5. In step S.sub.5, an appropriate duty factor Do is retrieved from 
a look-up table under consideration of the read intake air temperature 
T.sub.A and the read coolant temperature T.sub.C. This retrieved duty 
factor Do is outputted from the control means 103 in step S.sub.6. 
Therefore, a control signal with a fixed duty factor Do (e.g. 10 percent) 
is applied to the solenoid value 1F to make rich the mixture obtained 
through the carburetor 1 for prevention of engine overheat. 
FIG. 5 shows a range by a shaded portion within which a rich mixture having 
an air-to-fuel ratio lower than a theoretical ratio is obtained. In this 
drawing, a rich mixture is supplied to prevent engine overheat when intake 
air temperature exceeds 65.degree. C. and when engine coolant temperature 
exceeds 105.degree. C. simultaneously. 
In the above description, although a rich mixture is supplied under 
consideration of intake air and coolant temperatures, it is also 
preferable to supply a rich mixture for prevention of engine overheat when 
the transmission gear shift lever is set to a position other than Drive 
positions or when the vehicle speed is zero, because the engine may be 
overheated when the vehicle is at rest. Further, it is also possible to 
control ignition timing in the direction that combustion temperature is 
reduced in addition to the above-mentioned air-to-fuel ratio control. 
Furthermore, it is also preferable to inhibit the above-mentioned control 
for supplying a rich mixture when the engine is being started or being 
idled. 
As described above, in the air-to-fuel ratio control system for an engine 
according to the present invention, since air-to-fuel ratio is controlled 
so as to obtain a rich mixture when intake air temperature T.sub.A and 
engine coolant temperature T.sub.C both exceed respective reference 
values, it is possible to prevent engine overheat at high temperatures. 
It will be understood by those skilled in the art that the foregoing 
description is in terms of a preferred embodiment of the present invention 
wherein various changes and modifications may be made without departing 
from the spirit and scope of the invention, as set forth in the appended 
claims.