Engine air/fuel ratio sensing device

An engine air/fuel ratio sensing device for measuring the oxygen partial pressure or concentration in the exhaust gas of an engine. The device has a sensor and a control circuit coupled to each other. The sensor consists of an electrolyte oxygen pump cell and an electrolyte oxygen sensor cell, both cells having a gap portion therebetween. The oxygen pump cell pumps oxygen into ambient gas when electrically energized. The sensor cell produces an electromotive force when there is an oxygen partial pressure difference thereacross due to the pumping of the pump cell. The control circuit has a differential amplifier which receives as an input the electromotive force for comparison with a predetermined reference voltage. The amplifier continuously provides an output, used for driving a pumping current through the pump cell, until the electromotive force reaches the reference voltage at which an equilibrium condition for the pumping current is established. The differential amplifier accomplishes this function with a series combination of a feedback resistor and a feedback capacitor.

BACKGROUND OF THE INVENTION 
This invention relates to a device for measuring an oxygen concentration 
within an exhaust gas from an internal combustion engine etc., to sense 
the air/fuel ratio and in particular to an improved engine air/fuel ratio 
sensing device of an oxygen pump type constructed using an ion conductive 
solid electrolyte. 
It is hitherto well known in the art to control e.g. the engine of an 
automobile to run at a stoichiometric (theoretical) air/fuel ratio, by 
sensing its combustion state in relation to the stoichiometric air/fuel 
ratio according to the variation of an electromotive force produced by the 
difference of the oxygen partial pressure between the exhaust gas and the 
atmosphere, by means of an oxygen sensor constructed with an ion 
conducting solid electrolyte such as stabilized zirconia. It is to be 
noted here that air/fuel ratio (A/F) is given by the weight ratio of air 
to fuel and that the principle of oxygen sensing is described in "Applied 
Physics Lett. 38(5), Mar. 1, 1981". 
When the air/fuel ratio is the stoichiometric air/fuel ratio of 14.7, the 
above type oxygen sensor can provide a large output variation while 
outside the stoichiometric air/fuel ratio it provides a very low output 
variation. Therefore, when the engine is operated at an air/fuel ratio 
outside the stoichiometric air/fuel ratio, the output of such an oxygen 
sensor can not be utilized. 
There has already been proposed an air/fuel ratio sensor of an oxygen pump 
type which eliminates such a disadvantage and enables the engine to be 
operated at any air/fuel ratio. 
FIG. 1 shows an arrangement of an air/fuel ratio sensing device of an 
oxygen pump type, and FIG. 2 shows a cross sectional view of the sensor in 
FIG. 1 taken along line II--II, which is disclosed in a related 
application Ser. No. 606,926 filed May 4, 1984. 
In FIG. 1, within an exhaust pipe 1 of an engine (not shown) an air/fuel 
ratio sensor, generally designated by a reference numeral 2, is disposed. 
This sensor 2 is formed of a solid electrolyte oxygen pump cell 3, a solid 
electrolyte oxygen sensor cell 4, and a supporting base 5. The solid 
electrolyte oxygen pump cell 3 includes an ion conducting solid 
electrolyte (stabilized zirconia) 6 in the form of a plate having platinum 
electrodes 7 and 8 disposed on the respective sides thereof. The solid 
electrolyte oxygen sensor cell 4, likewise the pump cell 3, includes an 
ion conductive solid electrolyte 9 in the form of a plate having platinum 
electrodes 10 and 11 disposed on the respective sides thereof. The 
supporting base 5 supports the oxygen pump cell 3 and the oxygen sensor 
cell 4 so that they are oppositely disposed having a minute gap "d" of 
about 0.1 mm therebetween. 
An electronic control unit 12 is electrically coupled to the pump cell 3 
and the sensor cell 4. More specifically, the electrode 10 is connected 
through a resistor R1 to the inverting input of an operational amplifier A 
the non-inverting input of which is grounded through a DC reference 
voltage source V. This DC reference voltage serves to control the output 
voltage of the sensor cell 4 to assume said voltage V according to the 
oxygen partial pressure difference between those within the gap and 
outside the gap. The electrode 7 is connected through a resistor R0 to the 
emitter of a transistor Tr whose collector is grounded through a DC power 
source B and whose base is connected to the output of the operational 
amplifier A and the inverting input of the operational amplifier A through 
a capacitor C. The electrodes 8 and 11 are grounded. 
In operation, when the oxygen partial pressure within the gap portion 
between the cells 3 and 4 is the same as the oxygen partial pressure 
outside the gap portion, the sensor cell 4 generates no electromotive 
force. Therefore, the inverting input of the operational amplifier A 
receives no voltage and so the operational amplifier A provides as an 
output a maximum voltage corresponding to the reference voltage V to the 
base of the transistor Tr. Therefore, the transistor Tr is made conductive 
to cause a pump current Ip to flow across the electrodes 7 and 8 of the 
pump cell 3 from the voltage source B. Then the pump cell 3 pumps oxygen 
within the gap portion into the exhaust pipe 1. As a result, the sensor 
cell 4 develops an electromotive force thereacross according to the oxygen 
partial pressure difference on both sides of the cell 4. 
Therefore, the oxygen sensor cell 4 applies an electromotive force "e" 
generated across the electrodes 10 and 11 to the inverting input of the 
operational amplifier A through the resistor R1. The operational amplifier 
A provides an output now proportional to the difference between the 
electromotive force "e" and the reference DC voltage V applied to the 
non-inverting input. The output of the operational amplifier A drives the 
transistor Tr to control the pump current Ip. 
Thus, the electromotive force "e" approaches the reference voltage V. 
Accordingly, the control unit 12 reaches an equilibrium state and serves 
to provide a pump current Ip necessary for keeping the electromotive force 
"e" at the predetermined reference voltage V. The resistor R0 serves to 
provide an output corresponding to the pump current Ip supplied from the 
DC power source B as a pump current supply means. The pump current Ip 
corresponds to an air/fuel ratio value. This pump current Ip is converted 
into the voltage by the resistor R0 and is sent to a fuel control unit 
(not shown) so that the fuel control unit is controlled at a desired 
air/fuel ratio. The resistance of the resistor R0 is selected so as to 
prevent the pump current Ip from flowing excessively from the DC power 
source B. The capacitor C forms an integrator associated with the 
operational amplifier A and serves to make the electromotive force "e" 
precisely coincident with the reference voltage V. 
One example of the static characteristics of a conventional air/fuel ratio 
sensing device of an oxygen pump type thus constructed in the form of a 
negative feedback control is shown in FIG. 3. The different characteristic 
curves a and b are obtained by changing the reference voltage V in FIG. 1, 
as disclosed in related application Ser. No. 606,910 filed May 4, 1984. 
The characteristic curve a is preferable when the air/fuel ratio (A/F) is 
controlled in a so-called "rich" region where the A/F ratio is below the 
stoichiometric A/F ratio 14.7 and in a so-called "lean" region where the 
A/F ratio is above the stoichiometric A/F ratio 14.7 while the 
characteristic curve b is preferable when the A/F ratio is controlled at 
the stoichiometric A/F ratio 14.7. 
The air/fuel ratio sensor 2 illustrated in FIG. 1 has basically excellent 
characteristics because in either the rich region or the lean region the 
A/F ratio is linearly interrelated with the pump current Ip to thereby 
enable the engine to be operated at any A/F ratio. 
However, even with the sensor 2 in FIG. 1, the oxygen pump cell 3 and the 
oxygen sensor cell 4 have at least an electrical first order lag, 
respectively. There is also a considerable lag time required for measured 
gas within the exhaust pipe 1 to disperse into the minute gap "d". 
Furthermore, since the air/fuel ratio sensing device shown in FIG. 1 always 
requires a negative feedback control through an integrator in order to 
properly make the electromotive force "e" of the oxygen sensor cell 4 
coincident with the reference voltage V, it has the following 
disadvantages in the dynamic characteristic range. These disadvantages are 
also present with, the device shown in Hetrick, U.S. Pat. No. 4,272,329 
which includes an integrator formed by an operational amplifier and 
capacitor. 
First of all, the response lag as an air/fuel ratio sensor is always high 
due to the integration by the integrator so that its performance as 
required for the A/F ratio control of an engine is not totally 
satisfactory. 
Secondly, there is at least a phase lag of 180 degrees due to lag elements 
such as the oxygen pump cell 3 (90 degrees) and the sensor cell 4 (90 
degrees) and in addition the phase lag of 90 degrees due to the integrator 
formed of the operational amplifier A and the capacitor C, with the result 
that the sensor 2 has a danger of oscillation. Some experiments have 
revealed that the combination of various parameters such as the flow 
temperature of measured gas or an A/F ratio often causes the sensor to 
oscillate. 
SUMMARY OF INVENTION 
It is an object of the invention to provide an air/fuel ratio sensing 
device of an oxygen pump type with good response and improved 
characteristics by eliminating the above disadvantages. 
For this, briefly, the present invention has a differential amplifier the 
characteristics of which are formed by the addition of an integrator 
component and a gain component in the feedback circuit thereof. 
More specifically, the present invention provides an engine air/fuel ratio 
sensing device including an air/fuel sensor and a control circuit coupled 
to the sensor. The air/fuel sensor has a gap portion for introducing the 
exhaust gas of the engine, a solid electrolyte oxygen pump cell for 
controlling the oxygen partial pressure within the gap portion, and a 
solid electrolyte oxygen sensor cell for producing an electromotive force 
corresponding to the difference between the oxygen partial pressure in the 
exhaust gas within the gap portion and the oxygen partial pressure in the 
exhaust gas outside the gap portion. The control circuit has an input 
portion for comparing a predetermined reference voltage with the 
electromotive force to continuously provide an output until the 
electromotive force reaches the reference voltage and an output portion 
for driving a pump current through the oxygen pump cell in response to the 
output of the input portion, the input portion having a transfer function 
of an amplifier component plus an integrator component. As a result, the 
air/fuel ratio of the engine is detected according to an output 
corresponding to the pump current. 
The input portion comprises an input resistor one end of which is connected 
to one of the electrodes of the sensor cell, a differential amplifier the 
inverting input of which is connected to the other end of the input 
resistor and the non-inverting input to which is connected a source of the 
predetermined reference voltage, and a feedback series circuit consisting 
of a feedback resistor and a feedback capacitor, the feedback circuit 
being connected between the inverting input and the output of the 
differential amplifier. The transfer function is given by R2/R1+1/sCR1, 
where R2 represents the resistance of the feedback resistor, R1 the 
resistance of the input resistor, s Laplacian, and C the capacitance of 
the feedback capacitor. 
The output portion comprises a transistor whose base is connected to the 
output of the differential amplifier and whose collector is grounded 
through a DC power source, and a current limiting resistor connected 
between the emitter of the transistor and one of the electrodes of the 
pump cell, the voltage drop of the current limiting resistor being used as 
an output of the control circuit means for a fuel control.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
One preferred embodiment of an air/fuel ratio sensing device for an engine 
in accordance with the present invention will now be described in detail 
with reference to the accompanying drawings, particularly FIG. 4. 
As seen from the comparison of the arrangements of FIGS. 4 and 1, they are 
different only in that the former has an electronic control circuit unit 
20 including a resistor R2 serially connected with the capacitor C in the 
feedback circuit of the operational amplifier A of the latter. 
Therefore, the transfer function G of the circuit formed of the operational 
amplifier A, the resistors R1 and R2, and the capacitor C is given by the 
following equation: 
EQU G=R2/R1+1/sCR1 
where s represents Laplacian. 
In this equation, the first term R2/R1 represents the voltage gain of the 
output to the input of the operational amplifier A, the output voltage 
being proportional to the input voltage which is the voltage difference of 
the electromotive force "e" of the oxygen sensor cell 4 and the reference 
voltage V. The second term 1/sCR1 represents the integrated value of the 
input voltage of the operational amplifier A. It is to be noted that the 
transfer function of the oxygen sensing device in FIG. 1 (disclosed in 
related application Ser. No. 606,926, filed May 4, 1984 ) is given by the 
second term alone. 
Apparent from the above equation, the transfer function G approximates to 
R2/R1 in a higher frequency region of the electromotive force e, which 
includes an AC component superposed with a DC component, out of the sensor 
cell 4 while it approximates to 1/sCR1 in a lower frequency region 
thereof. Namely, in the lower frequency region of the electromotive force 
e, the control device according to the present invention serves as an 
integrator like the control device in FIG. 1 to properly control the 
voltage difference at the inputs of the operational amplifier A to zero, 
resulting in the above noted equilibrium state. 
On the other hand, in the higher frequency region of the electromotive 
force "e" of the sensor cell 4, the control device in FIG. 4 provides a 
sufficiently high gain defined by R2/R1 and good response because it 
serves as a differential amplifier which has no phase lag. 
Therefore, it is made possible to perform good control because of this good 
responsiveness if a sensing device according to the present invention as 
shown in FIG. 4 is employed for making an air/fuel ratio control for an 
engine. 
Furthermore, since in the higher frequency region the sensing device 
according to the present invention is not required to function as an 
integrator requiring a phase lag of 90 degree, the phase lag in the higher 
frequency region no longer exceeds 180 degrees in a normal condition and 
so the sensing device according to the present invention has little danger 
of the above noted oscillation tendency. It is to be noted that 
experminents have shown that such an oscillation does not arise and good 
characteristics are provided in any possible combination of engine 
parameters such as flow rate, temperature, or air/fuel ratio of a measured 
gas. 
In accordance with the present invention as stated above by the combination 
of an element sensing as an integrator of the characteristics of the 
differential amplifier with another element generating a proportional 
output voltage, an air/fuel ratio sensor can be achieved having the 
stabilized characteristics of good responsiveness, no loss of control for 
keeping the EMF of the oxygen sensor correctly following a predetermined 
reference voltage and further has no fear of oscillation. Also just by the 
addition of only one element (resistor R2) to the arrangement shown in 
FIG. 1, the desirable characteristics can be obtained extremely simply. 
Furthermore, it is also possible that the integrator be provided 
independently of the proportional amplifier and adding their respective 
outputs by an adder. 
It will be appreciated by anyone of ordinary skill in the art that the 
invention should not be limited to the described and illustrated 
embodiment but various modifications are possible without departing from 
the spirit of the invention.