Compensation for inherent fluctuation in output level of exhaust sensor in air-fuel ratio control system for internal combustion engine

In a feedback control system for maintaining the air-fuel ratio of a combustible mixture fed to an internal combustion engine at a preset ratio, a fluctuation in the output characteristic of an exhaust sensor due to deterioration or low temperature is compensated for by varying a reference voltage, which serves as a standard of comparison, in response to a change in a maximal value of the sensor output voltage.

This invention relates to a feedback control system for maintaining the 
air-fuel ratio of a combustible mixture fed to an internal combustion 
engine at a preset ratio, which system is of the type having an exhaust 
sensor for estimating a realized air-fuel ratio, and more particularly to 
a method of compensating for an inherent fluctuation in the output 
characteristic of the exhaust sensor by establishing a reference input 
signal in the feedback control system, the amplitude of which reference 
signal is variable according to a change in the output characteristic of 
the exhaust sensor, and an electrical circuit for accomplishing the 
method. 
In internal combustion engines, it is important for the reduction of the 
concentration of pollutants in the exhaust gas to maintain the air-fuel 
ratio of a combustible mixture fed to the engine exactly at an optimumly 
preset ratio. As is well known, the air-fuel ratio realized in the engine 
can be estimated by detecting the concentration of a certain component of 
the exhaust gas (which may be O.sub.2, CO, CO.sub.2, HC or NOx), and 
various types of exhaust sensors for this use are now available. In known 
feedback control systems for precisely controlling the air-fuel ratio, a 
control signal for regulating either the fuel feed rate or the air feed 
rate in an air-fuel proportioning device, for example a carburetor or a 
fuel injection system; is typically produced in the following manner. Any 
deviation of the output of an exhaust sensor from a preset reference 
signal (which corresponds to the preset air-fuel ratio) is detected in a 
deviation detection circuit (for example, a differential amplifier or a 
comparator), and the control signal is produced by either multiplying or 
integrating the detected deviation, or alternatively by the addition of 
the multiplied deviation (a proportional component of the control signal) 
to the integrated deviation (integral component). 
The control signal is produced in the above described manner on the premise 
that the output of the exhaust sensor has a definite correlation with the 
air-fuel ratio of the combustible mixture consumed in the engine. However, 
practical exhaust sensors inevitably exhibit changes in their output 
characteristic when exposed to various temperatures and/or used for a long 
period of time, because the exhaust sensors have either a semiconductor or 
an electrolyte as the sensing element. When the relationship between the 
air-fuel ratio of the combustible mixture and the output of the exhaust 
sensor is different from a preliminarily calibrated one while the 
reference signal is maintained constant, the application of the control 
signal to the air-fuel proportioning device results in the regulation of 
the air-fuel ratio to a ratio unwantedly deviated from the preset ratio. 
With respect to a feedback control system for maintaining the air-fuel 
ratio of a combustible mixture fed to an internal combustion engine to a 
preset ratio, which system includes an exhaust sensor capable of producing 
an electrical output representing the concentration of a component of the 
exhaust gas which concentration is correlated to the air-fuel ratio 
realized in the engine, it is an object of the present invention to 
provide a method of compensating for an inherent fluctuation in the output 
characteristic of the exhaust sensor by establishing a reference signal, 
which reference signal serves as a standard of comparison in detecting any 
deviation of the amplitude of the output of the sensor from an expected 
amplitude corresponding to the preset air-fuel ratio and automatically 
fluctuates in its amplitude in response to a change in the relationship 
between the aforementioned concentration and the amplitude of the output 
of the exhaust sensor. 
It is another object of the invention to provide an electrical circuit as 
part of the above described feedback control system for establishing a 
reference signal according to a method of the invention. 
According to a method of the invention, an inherent fluctuation of the 
output characteristic of the exhaust sensor is compensated for by varying 
the reference voltage in response to and in a definite correlation with a 
change in a maximal value of the output voltage of the exhaust sensor. 
The reference voltage can be varied by continuously applying the output 
voltage of the exhaust sensor to a maximal input retention circuit having 
a capacitor and a voltage divider which is adjusted such that the 
reference voltage is continuously in a definite proportion to a maximal 
value of the output voltage of the exhaust sensor. Alternatively, the 
reference voltage is varied stepwise by varying the resistance of a 
resistor for developing the reference voltage when a comparator detects 
that the maximal value of the output voltage of the exhaust sensor has 
lowered to a predetermined voltage. 
A variable reference voltage producing circuit according to the invention 
has a capacitor to which the output voltage of the exhaust sensor is 
continuously applied through a diode, preferably with the provision of a 
preamplifier for providing a high input impedance to the circuit, and a 
voltage divider in parallel with the capacitor. The proportion of the 
reference voltage to the maximal value of the output voltage of the 
exhaust sensor can be determined by the adjustment of the voltage divider. 
When the exhaust sensor is of the type which exhibits a great lowering in 
the maximal value of the output voltage at low temperatures, the reference 
voltage producing circuit preferably includes an auxiliary circuit which 
continuously produces a constant voltage below the maximum value of the 
output of the voltage divider so that the constant voltage may serve as 
the reference voltage when the output of the voltage divider is below the 
constant voltage. 
A circuit for varying the reference voltage stepwise has a first circuit 
having a voltage divider for producing the reference voltage which 
alternatively has two different magnitudes, a switching circuit including 
a flip-flop for governing the resistance of the voltage divider, a 
comparator for maintaining the switching circuit and the voltage divider 
in a first state when the maximal value of the output of the exhaust 
sensor is above a predetermined voltage, a second circuit having a 
capacitor which also receives the output of the comparator and a 
resistance, and another comparator the output of which causes the 
flip-flop to take a second state and shifts the reference voltage to a 
lower magnitude when the output voltage of the second circuit lowers to a 
predetermined voltage.

With respect to an internal combustion engine indicated at 10 in FIG. 1, an 
air-fuel ratio control system, which is the object of the invention, 
includes a controllable air-fuel proportioning device 12 exemplified by a 
carburetor or a fuel injection system, an exhaust sensor 14 installed in 
the exhaust line 16 of the engine 10, an electrical circuit 18 for 
producing a reference signal, another electrical circuit 20 exemplified by 
a differential amplifier or a comparator arranged to receive the output of 
the exhaust sensor 14 and the reference signal and produce an output 
signal representing the magnitude of the deviation of the output of the 
exhaust sensor 14 from the reference signal, and a control circuit 22 
which produces a control signal for the control of the air-fuel 
proportioning device 12 by modulating the output signal of the comparison 
circuit 20 in a manner as hereinbefore described. In conventional air-fuel 
ratio control systems of the illustrated type, the circuit 18 has merely 
the function of providing a constant reference voltage to the comparison 
circuit 20, so that the output of the exhaust sensor 14 is not applied to 
this circuit. According to the invention, the output of the exhaust sensor 
14 is applied to both the comparison circuit 20 and the reference signal 
producing circuit 18 as will hereinafter be described in detail. 
At present, a most familiar example of the exhaust sensor 14 is an oxygen 
sensor which is essentially an oxygen concentration cell having a solid 
electrolyte, for example, of a stabilized zirconia system. When such an 
oxygen sensor is used as the exhaust sensor 14 in the control system of 
FIG. 1 and the engine 10 is a gasoline engine, the output voltage of the 
oxygen sensor varies as represented by the curve (A) in FIG. 2 as the 
air-fuel ratio (by weight) of the combustible mixture consumed in the 
engine 10 varies. In many cases, the control system will be adjusted to 
maintain the air-fuel (gasoline) ratio at the stoichiometric ratio which 
is about 14.8. Since the output voltage of the oxygen sensor is 0.5 V when 
the air-fuel ratio is 14.8, a 0.5 V signal may constantly be applied to 
the comparison circuit 20 in order to correct any deviation of the 
air-fuel ratio from 14.8. 
However, the output characteristic of the oxygen sensor shifts from the 
curve (A) to a different curve (B) when the sensor is exposed to the 
exhaust gas for a prolonged period of time. On the curve (B), the output 
voltage for air-fuel ratios below a point near the stoichiometric ratio is 
lower than that of the curve (A). A similar lowering of the output voltage 
occurs also when the oygen sensor is used at relatively low temperatures 
because of a noticeable increase in the internal resistance of the sensor 
or concentration cell. If the amplitude of the reference signal is kept at 
0.5 V even though the output characteristic of the oxygen sensor has 
varied as represented by the curve (B), the air-fuel ratio control system 
fails to maintain the air-fuel ratio at 14.8 as intended: the air-fuel 
ratio is regulated to a lower ratio indicated at x in FIG. 2. 
According to the invention, the reference signal is not a constant voltage 
signal but a variable voltage signal whose amplitude has a definite 
relation with a maximal value of the output of the exhaust sensor 14. With 
respect to the oxygen sensor having an output characteristic as shown in 
FIG. 2, the maximal value of the output is about 1.0 V when the sensor is 
used in an optimum state. If the reference signal is produced to always 
have an amplitude equal to 1/2 of the maximal value of the output of this 
oxygen sensor, the air-fuel ratio can be regulated to 14.8 while the 
output characteristic of the oxygen sensor is as represented by the curve 
(A). When the output characteristic of the oxygen sensor is a represented 
by the curve (B) (the maximal value has lowered from about 1.0 V to about 
0.8 V), the amplitude of the reference signal lowers from 0.5 V to about 
0.4 V. As the result, the air-fuel ratio is regulated to a ratio y which 
is closer to 14.8 than the ratio x is. Since the relationship between a 
maximal value of the output of the exhaust sensor 14 and the amplitude of 
the reference signal can optionally be determined, it is possible to make 
the ratio y closer to the intended air-fuel ratio (14.8) than as is 
illustrated. Alternatively, the amplitude of the reference signal may be 
varied in dependence on the mean value of maximal and minimal values of 
the output of the exhaust sensor 14 as will be illustrated later. 
FIG. 3 shows an example of the construction of the circuit 18 for producing 
a variable reference signal in the case when it is intended to 
continuously vary the amplitude of the reference signal with a change in a 
maximal value of the exhaust sensor 14. 
The output of the exhaust sensor 14, for example an oxygen sensor of the 
above described type, is applied to both the negative input terminal of a 
comparator 21 (which serves as the comparison circuit 20 in FIG. 1) and 
the reference signal producing circuit 18. This circuit includes a maximal 
input retention circuit 18a which is fundamentally constituted of a diode 
24, a capacitor 26 and a voltage divider 28 having two resistors 28a and 
28b in parallel with the capacitor 26. In addition, an operational 
amplifier 30 of the voltage follower connection type is included as the 
entrance to this circuit 18 to provide a high input impedance to this 
circuit 18 so that the output of the exhaust sensor 14 may be applied to 
the comparator 21 without being influenced by the circuit 18. The output 
of the operational amplifier 30 is applied to the diode 24 via a 
transistor 32 which is employed as a temperature compensation means for 
the diode 24. 
The retention circuit 18a in the circuit 18 of FIG. 3 can retain a maximal 
value of an input (in this case the output of the exhaust sensor 14) and 
provide an output whose amplitude is in definite proportion to the maximal 
value of the input. The proportion of the amplitude of the output of the 
circuit 18 to the maximal value of the input is determined by the ratio of 
the resistance R.sub.1 of the resistor 28a to the resistance R.sub.2 of 
the resistor 28b. The output of the circuit 18 has an amplitude equal to 
1/2 of the maximal value of the input when R.sub.1 = R.sub.2. The output 
of the circuit 18 is applied to the positive input terminal of the 
comparator 21 as a reference signal, so that any fluctuation in the 
maximal value of the output of the exhaust sensor 14 can be compensated 
for to a desired extent by a simultaneous fluctuation in the amplitude of 
the reference signal. 
It will be apparent that the voltage divider 28 may be replaced by a 
variable resistor (not shown). 
FIG. 4 shows a different construction of the reference signal producing a 
circuit 18. The amplitude of a reference signal produced by this circuit 
18A varies stepwise when the maximal value of the exhaust sensor 14 
fluctuates to a certain extent. 
The circuit 18A has a first comparator 34. The output of the exhaust sensor 
14 is applied not only to the negative input terminal of the comparator 21 
but also to the positive input terminal of this comparator 34. As a 
standard of comparison, a constant reference voltage, which is developed 
by impressing a constant voltage Vcc on a resistor 35, is applied to the 
negative input terminal of the first comparator 34. This reference voltage 
is lower than the maximal value of the output of the exhaust sensor 14 in 
a normal or optimum state: for example, 70% of the maximal value. A 
voltage divider 40 of the circuit 18A has two resistors 40a and 40b and is 
imposed with the constant voltage V.sub.cc to provide an output voltage as 
a reference signal to the comparator 21. A transistor 38 is connected in 
parallel with one (40a) of the two resistors 40a and 40b so that the 
connected resistor 40a may be by-passed when the transistor 38 is in the 
conducting state. A flip-flop 36 is arranged to receive the output of the 
first comparator 34 and apply its Q output to the base of the transistor 
38. When the exhaust sensor 14 exhibits a normal output characteristic 
(the maximal value of the output is greater than the reference voltage 
provided by the resistor 36), the output of the first comparator 34 takes 
the form of a logic "1" signal. Accordingly, the flip-flop 36 is in the 
set state, so that the Q output is a "0" signal. In this state, the 
transistor 38 is in the conducting state and makes the resistor 40a 
ineffectual. The reference signal developed by the voltage divider 40, 
therefore, has a higher one of two alternatively realizable levels of 
amplitudes: for example, the amplitude of the reference signal in this 
state may be 0.5 V with respect to the above described oxygen sensor. 
The circuit 18A has a retention circuit 18b which consists of the diode 24, 
capacitor 26 and a resistor 42 in parallel with the capacitor 26. The 
output of the first comparator 34 is applied also to this retention 
circuit 18b, and the output of the retention circuit 18b is applied to the 
negative input terminal of a second comparator 44. The constant voltage 
V.sub.cc is imposed on a resistor 46 to develop a constant reference 
voltage, which is below the maximal value of the output voltage of the 
exhaust sensor 14 and is applied to the positive input terminal of the 
second comparator 44. The output of the second comparator 44 is applied to 
the flip-flop 36 so that the flip-flop 36 may be reset when the second 
comparator 44 provides an "1" output signal. While the output voltage of 
the retention circuit 18b is higher than the reference voltage developed 
across the resistor 46, the second comparator 44 provides a "0" output 
signal. Accordingly, the flip-flop 36 remains in the set state and the 
reference signal produced by the voltage divider 40 is kept at the higher 
level even if the exhaust sensor 14 exhibits a slight lowering in the 
maximal value of its output. 
When the maximal value of the output of the exhaust sensor 14 becomes below 
the reference voltage produced by the resistor 36, the first comparator 34 
continuously provides a "0" output signal. If the retention circuit 18b 
continues to receive the "0" output signal from the first comparator 34 
for a certain period of time, the output voltage of the retention circuit 
18b becomes below the reference voltage produced by the resistor 46 due to 
discharge of the electric charge stored in the capacitor 26. Then the 
second comparator 44 produces an "1" output signal and the flip-flop 36 is 
reset. Accordingly the Q output of the flip-flop 36 becomes an "1" signal 
and the transistor 38 is cut off. Consequently the amplitude of the 
reference signal produced by the voltage divider 40 falls to a lower level 
(for example, 0.35 V compared with the higher level of 0.5 V) determined 
buy the two resistors 40a and 40b. 
The circuit 18A preferably includes a warning circuit consisting of a 
resistor 48, a transistor 50 and an indicator lamp 52. The Q output of the 
flip-flop 36 is applied to the base of the transistor 50 through the 
resistor 48. Accordingly the transistor 50 is in the conducting state and 
the lamp 52 is lighted when the Q output is an "1" signal, i.e. when the 
lowering of the output of the exhaust sensor 14 is more than tolerable. 
By applying a variable magnitude reference signal produced by the above 
described method to the comparator 21 or the comparison circuit 20 in FIG. 
1, it is possible to accomplish a precise control of the air-fuel ratio by 
means of a control system constructed generally as shown in FIG. 1 even 
though the exhaust sensor 14 is either deteriorated to a certain extent by 
lapse of time or exposed to a low temperature exhaust gas. 
If the exhaust sensor 14 is a conventional oxygen sensor when the amplitude 
of the reference signal is allowed to continuously vary as described with 
reference to FIG. 3, there is a problem that the reference signal will 
have an extremely low amplitude when the exhaust gas temperature is very 
low as experienced at cold starting of the engine 10. This problem arises 
from the fact that conventional oxygen sensors have a very high internal 
impedance unless maintained at sufficiently high temperatures. Besides, 
the comparison circuit 20 generally has a very high input impedance 
(usually on the order of megohm) and is connected to the exhaust sensor 14 
(oxygen sensor) with a harness of a considerable length. Accordingly the 
comparison circuit 20 chances to make a malfunction attributable to a 
noise, resulting in the instability of the air-fuel ratio, when the 
reference signal is of an extremely low amplitude. The reference signal 
producing circuit 18, therefore, preferably includes a circuit for holding 
the amplitude of the reference signal at a definite value while the 
maximal value of the output of the exhaust sensor 14 is below a 
predetermined value. 
A circuit 18B of FIG. 5 has the maximal input retention circuit 18a shown 
in FIG. 3 and, in addition, a minimal output holding circuit 18c. This 
circuit 18c consists of a voltage divider 54 having two resistors 54a and 
54b, a diode 56 through which the output of the voltage divider 54 can be 
applied to the positive input terminal of the comparator 21, and a 
transistor 58 arranged to serve as a temperature compensation means for 
the diode 56. The constant voltage V.sub.cc is imposed on the voltage 
divider 54, so that the output of this circuit 18c has a definite 
amplitude determined by the resistances of the two resistors 54a and 54b. 
The output of the maximal input retention circuit 18a is applicable to the 
positive input terminal of the comparator 21 through a diode 60, and a 
transistor 62 is provided as a temperature compensation means for this 
diode 60. 
In the circuit 18B of FIG. 5, the function of the maximal input retention 
circuit 18a is the same as in the case of FIG. 3. For example, the circuit 
18a and accordingly the circuit 18B provide a reference signal which is 
always 50% in amplitude of the output of the oxygen sensor 14 so long as 
the reference signal has an amplitude greater than the amplitude of the 
constant output of the circuit 18c. When the maximal value of the output 
of the oxygen sensor 14 is extremely low, for example less than 50% of the 
value in normal state, the amplitude of the output of the circuit 18B (the 
reference signal applied to the comparator 21) lowers no more but is held 
at the output voltage of the minimal output retention circuit 18c. FIG. 6 
shows the relationship between the maximal output voltage of the oxygen 
sensor 14 having the output characteristic of FIG. 2 and the amplitude of 
the reference signal produced by the circuit 18B, assuming that the output 
voltage of the maximal input retention circuit 18a is 50% of the maximal 
value of the output of the oxygen sensor 14 and that the minimal output 
retention circuit 18b produces a constant voltage of 0.2 V. 
Thus, the provision of the minimal output retention circuit 18b in the 
circuit 18B prevents the control system of FIG. 1 from errorneously 
functioning by the influence of a noise even when the maximal value of the 
output of the oxygen sensor 14 is extremely low. The circuit 18B has an 
additional advantage with respect to the operation of the engine 10 at low 
engine temperatures. It is desirable to temporarily feed the engine 10 
with a slightly fuel-enriched mixture (lower the air-fuel ratio) for 
securing the stability of the engine operation when the engine temperature 
is very low as in the case of cold starting of the engine 10, but the 
output voltage of the oxygen sensor 14 under such a low temperature 
condition is almost zero due to a great internal resistance and does not 
cause the air-fuel ratio control system to so act as to lower the air-fuel 
ratio. The minimal output retention circuit 18c, however, provides the low 
voltage reference signal in this case and causes the control system to 
lower the air-fuel ratio until the engine temperature of exhaust 
temperature rises to a sufficiently high level. 
A change in the output characteristic of the exhaust sensor 14 usually 
occurs as a lowering of a maximal value of the output voltage while a 
minimal value remains substantially unchanged, and in many cases the 
reference voltage for the comparison circuit 20 is preset around the 
middle of the total range of the output voltage of the exhaust sensor 14. 
Accordingly the reference voltage may be varied in dependence on the mean 
value of maximal and minimal values of the output voltage of the exhaust 
sensor 14. FIG. 7 shows a modification of the circuit of FIG. 3 to take 
the mean value as the indication of the output characteristic of the 
exhaust sensor 14. This circuit 18C includes all the elements of the 
circuit 18 of FIG. 3. In addition, a capacitor 64 is interposed between 
the voltage divider 28 and ground, and a diode 66 is connected to the 
voltage divider 28 in parallel with this capacitor 64. The cathode of this 
diode 66 is connected to the junction between the transistor 32 and the 
anode of the diode 24. Accordingly the transistor 32 serves as temperature 
compensation means for both the diodes 24 and 66. The voltage divider 28 
(which is an element of the maximal value retention circuit 18a), the 
capacitor 64 and the diode 66 constitute a minimal value retention circuit 
18d. The output of this circuit 18C has a variable amplitude in proportion 
to the mean value of the maximal and minimal values of the input signal 
amplitude by making the resistances of the two resistors 28a and 28b 
nearly equal.