Catalyst monitoring using ego sensors

A method of monitoring, while on board an automotive vehicle, one or more of catalyst performance, engine misfire, and combustion quality, the vehicle having an internal combustion engine equipped with a catalyst for converting noxious emissions of the engine, comprising: (i) exposing at least one pair of EGO sensors to substantially the same emissions either exiting from the engine or from the catalyst, one of the EGO sensors having its electrode highly catalytic, and the other sensor having its electrode low-to-noncatalytic; (ii) comparing the outputs of the sensor electrodes (amplitude, frequency, or phase shift) to determine if there is a sufficient differential to indicate a misfire or poor combustion in the case of the sensors being located downstream of the engine exhaust but upstream of the catalyst, or indicating poor catalyst efficiency in the case of the sensors being placed substantially immediately downstream of the catalyst. The catalyst may be a three-way catalyst (or an oxidation catalyst). The sensors may be of the EGO, HEGO, or UEGO types. Two pairs of sensors may be used, a first pair being placed substantially immediately upstream of the catalyst and the second pair being placed substantially immediately downstream of the catalyst, the pairs of EGO sensors being incorporated into a closed-loop feedback control of the engine fuel control system.

BACKGROUND OF THE INVENTION 
1. Technical Field 
This invention relates to the art of using exhaust gas oxygen (EGO) sensors 
for detecting catalyst failure, such catalysts being of the type that 
converts automotive engine emissions to non-noxious gases and water vapor. 
2. Discussion of the Prior Art 
There is growing concern that to improve air quality in the United States, 
emission related components, such as a catalyst, must be monitored on 
board the vehicle to determine any malfunction. Catalyst monitoring has 
been and still is the least understood, both conceptually and practically, 
of the emission related components. 
EGO sensors have been used in the past, in pairs, to monitor catalysts, one 
sensor being placed upstream from a catalyst and the other placed 
downstream of the catalyst, and the signals from each of such sensors were 
evaluated to determine any difference that would indicate the catalyst was 
degraded. It is presumed by such prior art that a properly operating 
catalyst would be capable of dampening the periodic rich to lean 
excursions resulting from the limit cycle A/F feedback control or 
intentionally generated in the exhaust stream and that a substantial loss 
in catalyst performance through loss in actual conversion activity and/or 
oxygen storage activity would result in a decrease in this dampening 
ability of the catalyst. This generalized approach to catalyst monitoring 
compares complex signal features from both devices, each of which is 
disposed in a different environment and exposed to different exhaust gas 
locations, and furthermore presumes that there is a correlation between 
catalyst oxygen storage sensed signal features, and catalyst performance. 
Often there is no such correlation. However, since each sensor uses a 
similar construction, including a catalytic coating that acts as a 
microcatalyst, failure based on the inability of the main catalyst to 
convert emissions may be hidden or masked by the sensor itself. 
Patented variations of the two sensor catalyst monitoring system have 
utilized or compared many sensor signal characteristics, including voltage 
amplitude, phase shift, and frequency ratioing. In some cases, an 
artificial change in the sensor signal is created by modulation of the 
engine A/F ratio which, it is hoped, will more clearly show the onset of 
catalyst degradation. Unfortunately, all of such prior art approaches have 
at least the following characteristics in common: they expose the 
electrodes of the sensors to different emission gases, the sensors 
inherently have construction variations in tolerances and aging, and a 
decision as to catalyst degradation cannot be made without comparison to 
an artificial reference. Such prior art sensor system approaches are 
inaccurate not only due to such sensor differences but also are not able 
to sense a difference in oxygen between equilibrated and nonequilibrated 
oxygen conversion or combustion. 
What is needed is a system that more reliably monitors catalyst degradation 
or inadequate engine combustion. 
SUMMARY OF THE INVENTION 
The invention uniquely deploys an EGO sensor's rapid ability to detect a 
gas mixture's difference from chemical equilibrium and not mask such 
ability by presuming the oxygen storage capability of a main catalyst must 
first be detected. This is a significant inversion of logic used by the 
prior art. 
The invention herein uses an approach different than prior art to detect 
either catalyst malfunction or engine misfire. The invention recognizes 
that an engine or catalyst each are gas mixture equilibrators. That is, a 
properly functioning catalyst or engine burns combustible intake gases or 
fluids to near chemical equilibrium. However, a standard EGO sensor also 
is an equilibrator because it uses catalytic electrodes and coatings to 
more fully combust or "equilibrate" either engine or catalyst exhaust 
gases to improve its stoichiometric control point sensing accuracy. If the 
sensed gases are already at or near equilibrium, catalytic electrode or 
coatings activity would not be necessary and would perform no function; it 
would be superfluous. 
The logic of this invention is based on controlled single factor variation. 
It follows below. Build nearly identical EGO sensor pairs which differ 
within a pair in that one sensor is fully catalytic while the other has 
reduced or no catalytic activity at its electrode or coating. Place such a 
nearly identical sensor pair in a common operating flow and gas 
environment downstream of either an engine or catalyst. If sensed gases 
are at or near equilibrium, there will not be a difference between the 
respective sensor outputs because a sensor's catalytic activity is not 
needed. If sensed gases are not at or near equilibrium (due to catalyst 
degradation or engine malfunction), there will be a difference between the 
sensors' outputs, because one sensor's catalytic activity is needed and is 
missing. The difference will occur because within a pair, one sensor's 
missing catalytic activity will actually be needed to bring gases to 
equilibrium. 
Thus, the invention places differentially catalyzed electrodes of oxygen 
sensors in either the exhaust gas exiting from the engine or in the 
exhaust gas exiting from the catalyst, and then compares signals generated 
by each of the electrodes and, if a predetermined difference is present, 
makes indication, respectively, of engine malfunction or catalyst 
degradation. Monitoring, while on board an automotive vehicle, can be 
carried out for one or more of catalyst performance, engine misfire, and 
combustion quality, the vehicle having an internal combustion engine 
equipped with a catalyst for converting noxious emissions of the engine. 
The catalyst may be a three-way catalyst or an oxidation catalyst. The 
sensors may be of the EGO, HEGO, or UEGO types. The sensors are used in 
pairs, a first pair being placed substantially immediately upstream of the 
catalyst and the second pair being placed substantially immediately 
downstream of the catalyst, the pairs of EGO sensors being incorporated 
into a closed-loop feedback control of the engine fuel control system. 
Both amplitude comparison, frequency change, or phase shift comparison may 
be used in detection of the difference in equilibrated and nonequilibrated 
gases passing by the sensors. 
This invention can be used when operating the engine under closed-loop 
control, first with one sensor of a pair in the feedback control, and then 
the other during a short monitoring period, i.e., less than 20 seconds; a 
change in the feedback signal is observed to obtain a determination of 
catalyst degradation. The change from one sensor to the other may be 
cyclically controlled at a repeating frequency to enhance reliability of 
the system. A correlation is made between resulting changes in the signal 
with switching frequency. 
Another aspect of this invention is the construction of a single sensor 
body having dual electrodes, one electrode being highly catalytic and the 
other being low-to-noncatalytic. The construction may have a common solid 
electrolyte zoned for two sensors by use of a barrier and two pairs of 
platinum electrodes, one of the pairs being exposed to an air reference 
cell and the other pair exposed to exhaust gases. One pair has the exhaust 
exposed electrode highly catalytic by use of a thin overcoating of porous 
platinum, and the other pair has the exhaust exposed electrode devoid of 
such coating or deactivated by lead or silver to produce a 
low-to-noncatalytic electrode. To simplify and make less expensive, the 
air reference may be eliminated from the construction and the 
differentially catalyzed electrodes placed on opposite sides of a 
non-zoned electrolyte thereby immersing the entire device in the exhaust 
gas.

DETAILED DESCRIPTION AND BEST MODE 
It is conventional wisdom in the art that an oxygen sensor will be able to 
detect a change in oxygen storage capacity of a catalyst and thereby 
presumably detect catalyst efficiency. Attempts to implement this wisdom 
have used a conventional oxygen sensor placed downstream of the main 
catalyst, but it also functions as a catalyst, more accurately a 
microcatalyst, because its electrode, exposed to the exhaust gases, is 
highly catalytic in accordance with conventional construction. It is 
theorized that when the main catalyst degrades, it cannot cyclically store 
oxygen. Thus, the EGO sensor will provide a switching signal increased in 
frequency and/or increased in amplitude. However, the actual signal must 
be compared to a reference signal or library of reference signals to judge 
whether the main catalyst is degraded. Matching reference signals properly 
may lead to erroneous results because of the variable instantaneous 
conditions within the system. Amplitude changes are unreliable as an 
indicator of catalyst degradation because such changes are caused by 
changes in the oxygen storage of the catalyst, not necessarily by changes 
in its conversion efficiency. Also, some change in sensor output could be 
caused by changes in the response of the sensor itself. Frequency change 
may not be an indicator of a degraded catalyst for the same reasons given 
above. 
To avoid the need for reference signals, it has also been theorized that an 
artificial fuel pulse be used to exceed the storage capacity of the main 
catalyst; analysis of the sensed signal, before and after the pulse, tends 
to more clearly indicate a degraded catalyst without the need for 
reference signals. This approach may be difficult to implement because of 
the need to use the downstream sensor in a closed-loop engine control and 
maintain the exhaust gases within a desired window of air/fuel ratio 
optimum for main catalyst conversion efficiency. 
The invention herein avoids any reliance on a correlation between oxygen 
storage of the main catalyst and its efficiency. In the preferred 
embodiment, two differentially catalyzed EGO sensor electrodes, whether 
integrated along one common electrolyte or used in separate electrolyte 
constructions, are substituted for the conventional EGO sensor downstream 
of the main catalyst; simultaneous and instantaneous comparison of the 
actual signals from each of such electrodes provides very reliable proof 
as to the efficiency of the main catalyst. Unreliable reference signals 
are avoided, the engine control system is not disrupted to accommodate 
catalyst monitoring, and false conclusions from reliance on the catalyst's 
ability to store oxygen is avoided. 
Essentially, the method of this invention comprises two main steps. The 
first step is to expose differentially catalyzed electrodes of an oxygen 
sensing system to essentially the exhaust gases from an engine (both being 
either upstream or downstream of the main catalyst, designed to convert 
all of such exhaust gases). The second step compares the signal outputs 
from such electrodes for an indication of a specific malfunction with 
respect to engine misfire, combustion deficiency, or main catalyst 
degradation (see FIG. 1). 
System Usage 
A first aspect of this invention is concerned with how the differentially 
catalyzed sensor electrodes, exposed to the exhaust gases, are used in a 
catalyst monitoring system. As shown in FIG. 2, a closed-loop feedback 
control can be employed having a primary feedback loop 10(a) and an 
enhancement feedback loop 10(b). In the primary feedback loop, a 
conventional EGO sensor 11 is disposed in the emission flow 12 from an 
engine 13 (upstream of the catalyst), the signal from the EGO sensor 11 
being connected to a feedback controller 14 which in turn supplies control 
information to an on-board computer or base fuel calculation means 16. 
Means 16 transmits a command signal to a fuel injector driver 17, the 
command signal controlling the pulse-width converter of the injector 
driver. There may be several injector drivers to accommodate each of the 
combustion cylinders of the engine, each of which must receive fuel pulses 
to carry out combustion therein within the engine in combination with 
inlet air 18 supplied to the engine. The signal from the first EGO sensor 
11 may be modified by voltage follower (comparator) 15 which reshapes the 
signal from a sine-like wave to essentially a square wave thereby 
alleviating the very high impedance of the sensor output. To enhance the 
feedback control loop, it may further contain adaptive tables 20 to 
provide more precise calculation of A/F ratios during dynamic conditions 
where the feedback system cannot respond rapidly enough. The on-board 
computer or fuel calculation means 16 also receives information of mass 
airflow from a device 19. The controller 14 is preferably a 
proportional-integral type wherein the coefficients of 
proportional-integral terms of a control algorithm are adjusted to a 
different gain. Gain is the slope of the signal output to the signal input 
(essentially its strength). The gain of the signal directly from the 
sensor is extremely high at the switch point and thus would lead to 
erratic adjustments if such signal was not modified with respect to its 
gain. 
To provide for enhanced feedback control and catalyst monitoring, the 
secondary control loop 10(b) deploys a second EGO sensor 30, having a 
highly catalytic electrode 30(a) exposed to the exhaust gases, and a third 
EGO sensor 31, having a low-to-noncatalytic electrode 31(a) exposed to the 
exhaust gases; sensors 30 and 31 are arranged for alternate connections to 
the feedback controller 14. The second and third EGO sensor electrodes 
that are exposed to the exhaust gases may be combined as a single split 
sensor construction placed after the catalyst to perform the monitoring. 
Such a split EGO sensor would actually be two EGO sensors in one; one of 
the EGO sensors would have a highly catalytic coating and the other would 
have no or little catalytic coating. 
Each signal from the second and third sensor electrodes 30(a) and 31(a) are 
respectively modified by a separate voltage follower or comparator 32, 33. 
The voltage follower is useful because the signal emanating from the 
sensor itself has very high impedance. The signal from the follower is 
then subjected to a low-gain modifier 34 or integrator. Thus, the signal 
is developed at an output that increases with time at a constant rate or 
that decreases at a constant rate to vary the pulse width of the air/fuel 
ratio controller in a closed-loop manner. The low-gain modifier switches 
from an increasing ramp to its decreasing ramp and back again in response 
to the output of the follower or comparator, which can be either one of 
two levels. The comparator changes or switches levels at a point where the 
waveform voltage of an oxygen sensor exceeds a reference voltage input to 
the comparator. The reference voltage input to the comparator is 
preferably known to be a voltage that will provide a uniform result even 
in conditions where the sensor waveform ages. 
The signal may further be modified by a bias adjust 35. The bias adjust is 
useful to compensate for dislocation of the air/fuel ratio signal to the 
lean side due to a slow change of partial pressure of oxygen, even when at 
the stoichiometric point of the sensor. This bias adjust moves the 
air/fuel ratio back to the proper window. 
The enhanced feedback system uses the highly catalytic electrode 30(a) (or 
second sensor) in normal mode to provide the enhanced feedback control. 
The low-to-noncatalytic electrode 31(a) (or third sensor) is alternately 
switched into the enhanced feedback system control and a comparison is 
made between the signals from such differentially catalyzed electrode(s) 
(sensors). A switch 36 is interposed into the enhanced feedback loop 
preferably after the comparators 32, 33. Although the switch 36 may be 
disposed at other locations in the signal connection between the sensors 
and the controller, it is desirable at this location because it minimizes 
the use of redundant components. Comparison may be carried out by use of a 
detector 38 connected to the signal output of the controller 14 (sensing 
the A/F signal-LAMBSE) which in turn is interpreted by an indicator block 
39 to alert the driver to the desired malfunction of the catalyst. 
Comparison of the signal output from the controller is advantageous 
because it allows a more accurate determination of the degree of 
degradation of the catalyst as will be discussed later. 
Alternate usage schemes for the differentially catalyzed electrodes of an 
oxygen sensor are shown in FIGS. 3-6. In FIG. 3, the use of an upstream 
sensor to provide primary feedback control is eliminated (either during 
the detection test or during all engine operations). The highly catalytic 
electrode 40 of the downstream split sensor (or pair of sensors) may be 
used as the normal mode for feedback A/F control and the 
low-to-noncatalytic electrode 41 is used only during catalyst 
interrogation. It may be desirable to cyclically switch back and forth 
between the highly catalyzed and noncatalyzed sensor electrodes at some 
suitable frequency (rather than just switching once); the resulting 
changes in the feedback A/F signal are correlated with the switching 
signal frequency. This can be done by use of a repeater device 42 which 
promotes the cyclical switching. Such switching is done in order to obtain 
a catalyst monitoring signal that alternates between two values (during 
the catalyst testing interval) rather than a signal which just switches 
once. The potential advantage in doing this is that the procedure may 
provide more reliability in identifying marginally defective catalysts. 
The cyclical switching operation would only be performed during a 
designated catalyst monitoring interval such as for about 20 seconds. When 
the catalyst monitoring is not being performed, the highly catalyzed 
electrode or sensor 40 (rather than the low-to-noncatalyzed electrode or 
sensor 41) would be used in the feedback A/F control or feedback trimming 
enhancement to provide the maximum air/fuel control accuracy. The other 
elements modifying the A/F control signal may be the same as in FIG. 2 or 
simplified as shown in FIG. 3. Detection of a signal difference is here 
made prior to the low gain adjust 34 and bias adjust 35. Detection is by 
way of block 44 (which simply compares the difference between output from 
sensors 40 and 41 and produces a malfunction indication signal to 
malfunction indicator 45 when the difference is greater than a preset 
value corresponding to a bad catalyst. Preset value could be a function of 
speed and torque. 
As shown in FIG. 4, differentially catalyzed electrodes (or sensors) 46 and 
47 may be placed downstream of the catalyst in open loop, with upstream 
sensor 48 operating in closed loop to feed back oxygen sensing information 
for A/F control. A switching device 49 may be cyclically controlled by 
repeater as shown. Detection and indication of malfunction is made similar 
to FIG. 3. 
The broad concept of this invention does not depend on which type of 
combusting device is upstream of the differentially catalyzed electrodes 
(sensors). The concept can be used to detect misfire and slow or late burn 
of each of the cylinders contributing to improper engine combustion, which 
improper combustion may damage the main catalytic converter. Two sensors 
or one split sensor device can be located upstream of the main catalyst 
converter 21 but downstream of the engine exhaust manifold of the engine 
13. One EGO sensor (having only one type of catalyzed electrode), 
regardless of position in the exhaust stream, cannot readily detect 
improper combustion. But, differentially catalyzed sensors (electrodes) of 
this invention can do so readily. It has been discovered that the 
low-to-noncatalyzed sensor (electrode) will exhibit a decided change in 
frequency when there is an ignition misfire (that is to say, the 
noncatalyzed sensor will produce an output signal having high frequency 
components corresponding to the rate of the misfire); a 
low-to-noncatalyzed sensor (electrode) will also exhibit a change in 
amplitude when slow or late cylinder burn occurs. 
As shown in FIG. 5, sensor 50, having a highly catalytic electrode, and 
sensor 51, having a low-to-noncatalytic electrode, are placed downstream 
of engine 13 but upstream of catalyst 21. The sensor 50 is normally 
connected in closed-loop feedback A/F control of the engine. The signal 
from each of the sensors is fed to a detection block 52 which is effective 
in determining when there is a sufficient difference in frequency or 
amplitude to alert a malfunction indicator 53 of misfire or slow or late 
burn. 
The use of differentially catalyzed sensors (electrodes) may be used to 
detect both misfire and combustion malfunction as well as provide 
interrogation of the main catalyst for proper functioning (see FIG. 6). In 
this embodiment, the highly catalyzed electrode (sensor) 55 acts to 
provide the normal oxygen sensing for closed-loop feedback A/F control. 
The low-to-noncatalyzed sensor (electrode) 56 is continuously compared by 
way of a detecting block 57 to trigger a malfunction indicator 58 if 
justified. 
The downstream differentially catalyzed sensors (electrodes) 59 and 60 are 
used the same as in the embodiment of FIG. 2 to periodically interrogate 
the main catalyst 21 as to its efficiency. A repeater device 1 may be 
utilized to switch between each of the sensors (electrodes) 59 and 60 to 
make the comparison. Detector 62 and malfunction indicator 63 receive and 
operate on the signal received upstream of low gain block 34 and bias 
adjust 35. 
Comparing Signals 
This invention uses differentially catalyzed electrodes (sensors) that 
allow the monitoring system to be specific to combustion (whether 
performed by the engine or by the catalyst) while eliminating temperature 
and flow sensitivity and eliminating the distortion and interference 
inherent in absolute measurement from a single device. 
For catalytic monitoring, a standard oxygen sensor with a highly catalytic 
electrode exposed to the exhaust flow downstream of a main catalyst can 
exhibit a voltage signal that shows little change from the signal sensed 
by an upstream sensor with a highly catalytic electrode when the main 
catalyst is bad (see FIG. 7). Each sensor is seeing essentially the same 
type of unconverted exhaust gas and each sensor equilibrates such gas in 
essentially the same way. However, when the catalyst is good, there is a 
substantial difference in signal between the highly catalytic upstream and 
downstream sensors. This substantial change in signal may be attributed to 
the fact that a good main catalyst fully equilibrates the exhaust gases 
prior to the downstream sensor seeing such gases. However there is a 
decided change in signal in the downstream sensor, when highly catalytic, 
depending on whether the main catalyst is good or bad. 
Even more clear is the fact that a highly catalyzed sensor (electrode) 
characteristic is shifted rich compared to the noncatalyzed sensor 
(electrode) (see FIG. 8). A rich shift herein means that the catalytic 
sensor will produce a certain mid-range output voltage at an A/F ratio 
which is richer than the A/F ratio required for the noncatalytic sensor to 
produce the same output voltage. The amount of A/F shift is dependent on 
the catalytic activity (i.e., on the conversion efficiency) of the main 
catalyst. A goal of this invention is to be able to operate the engine 
under closed-loop control with first one sensor in the feedback control 
and then the other in the feedback control and observe the change in the 
engine A/F feedback signal (LAMBSE) while doing so. Since the two sensors 
will produce the same output voltage at a different A/F value, there will 
be a difference in the A/F feedback signal of the engine depending on 
which sensor is in control (see FIG. 9). The reason there is a shift in 
the A/F, depending on whether the catalyzed or low-to-noncatalyzed 
electrode is used, can best be understood by reference to FIG. 10. But a 
low-to-noncatalytic sensor, placed downstream of the main catalyst, will 
exhibit little amplitude change in signal between a good and bad catalyst. 
The exhaust gases are passing through the main catalyst equilibrated in 
the case of a good catalyst but essentially unconverted in the case of a 
bad catalyst. But since the sensor cannot itself equilibrate the gases, 
there is a saturation of the sensor and the signal appears as a stretched 
form of the good catalyst signal, possibly at different levels due to a 
different mean A/F of the modulating A/F signal. Thus, when a sensor is 
capable of equilibrating itself, it will exhibit a greater signal 
amplitude and/or frequency. Therefore, since the difference between the 
two sensor characteristics is a function of the catalyst conversion 
efficiency, the magnitude of the change in the A/F feedback signal 
(LAMBSE), which occurs when switching from one sensor to the other, can be 
used as an indicator of the catalyst condition. 
Although this invention comprehends detecting the signal of the 
differentially catalyzed electrodes (sensors) anywhere along the 
closed-loop circuit, it is preferred to detect the A/F ratio feedback 
signal (LAMBSE). This preference can be understood by reference to FIGS. 
12 and 13. 
During use of the highly catalyzed sensor (electrode), shown to the left of 
line 60 in FIG. 11, the voltage signal is relatively steady at a 
predetermined plateau 61. When the low-to-noncatalyzed electrode (sensor) 
is made operative by switching, the voltage signal (to the right of line 
60) will exhibit a difference if the main catalyst is bad. The voltage 
will abruptly rise to a new plateau 62 and gradually recede to its 
original plateau as the A/F controller readjusts the A/F ratio. To see a 
decided difference in signal, using the voltage data of FIG. 11, the 
comparison must be made rather quickly at a moment when the voltage has 
made a sharp move, which leads to inaccuracy because of its rapid change. 
If the comparison is made too slowly, i e., about 10-15 seconds, the 
voltage will have receded and little difference will remain. Furthermore, 
the plateau 62 will saturate at some limiting value for all catalysts 
having conversion efficiency below a certain value. 
In FIG. 12, a preferred signal comparison is illustrated. The A/F feedback 
signal is sensed This may be accomplished by taking the signal at a 
location 63 (A/F feedback signal), as shown in FIG. 2, as opposed to 
taking the voltage signal at a location 64, as shown in FIG. 3. As shown 
in FIG. 12, the feedback signal, using a low-to-noncatalyzed electrode, 
will gradually rise to a new plateau 65 over a period of 5-10 seconds if 
the main catalyst is bad. In the case of a good catalyst, the A/F feedback 
signal will remain substantially at the original plateau 66, essentially 
the same as for the highly catalyzed sensor (electrode). This enables an 
interrogation scheme whereby after about 5-10 seconds, from the time the 
highly catalyzed electrode is switched to the low-to-noncatalyzed 
electrode, a clear, definite signal comparison can be made, free from 
inaccuracies. 
An additional virtue of using the A/F feedback signal for detection (i.e., 
taken at 63) is that it permits determination of a degree of malfunction 
or efficiency. The amount of A/F shift (or .DELTA. A/F) is indicative of 
the degree of hydrocarbon conversion efficiency degradation of the main 
catalyst. Engine/dynamometer tests were performed using the system of FIG. 
2. Specifically, closed-loop A/F measurements were made with first the 
highly catalytic sensor and then the noncatalytic EGO sensor in control, 
and the differences between the closed-loop A/F for each situation was 
determined The tests were repeated using three different catalysts and the 
results were plotted as a function of hydrocarbon conversion efficiency as 
shown in FIG. 15. Examination of the results shown in FIG. 15 verify that 
the invention concept works as anticipated for the catalysts examined. 
However, when using the voltage signal, such as taken-at location 64, the 
amplitude or frequency gives no clue as to the degree of efficiency of the 
catalyst. 
The voltage signal obtained from the sensor when placed upstream and used 
to detect misfire of combustion malfunction, is illustrated in FIGS. 13 
and 14. A highly catalyzed electrode (sensor), as shown in FIG. 13, will 
exhibit a voltage variation that is roughly sinusoidal (for normal limit 
cycle operation) for both signals 67 and 68 during normal combustion and 
cylinder misfire conditions, respectively. However, when the 
low-to-noncatalyzed electrode (sensor) is activated, the voltage variation 
70, 71 differs significantly from the normal combustion (to the left of 
line 69) and cylinder misfire (to the right of line 69) as shown in FIG. 
15. The frequency is highly increased when a misfire occurs. Slow or late 
burn (other combustion malfunctions) will give rise to an amplitude change 
in the voltage signal of the low-to-noncatalyzed sensor (electrode). 
Sensor Construction 
FIG. 16 shows a consolidated view of both a highly catalyzed sensor 
construction on the top side 16(a), and at the bottom side 16(b),a sensor 
construction having a low-to-noncatalyzed electrode exposed to the exhaust 
gases. The construction of FIG. 16(a) has a thimble-like structure 
positioned in the exhaust system of the engine. The exhaust gases 70 from 
the manifold, including unburned hydrocarbons, oxides of nitrogen, and 
carbon, along with O.sub.2, are passed in Proximity to the oxygen sensor. 
The oxygen sensor 79 has a reference port 71 located within an insulator 
base 72 that receives ambient atmospheric gases comprised essentially of 
79% nitrogen and 21% oxygen in the form of O.sub.2. The oxygen sensor 79 
further comprises a solid electrolyte oxygen ion conductor 73 of ZrO.sub.2 
or the like which has an inner electrode 74 of some noble metal, 
preferably platinum. On the outer surface of the solid electrolyte 73 is a 
highly catalytic electrode 75 comprised preferably of a noble metal solid 
strip, such as platinum, with a painted dot of porous platinum 75(a). A 
protective oxide covering 76, in the preferred form of a porous coating of 
MgO.Al.sub.2 O.sub.3 spinel, overlays the entire outside active surface of 
the sensor 70. All the layers 73/74/75/76 are porous either to molecules 
or ions of oxygen; the two platinum conduction layers 74/75 and terminals 
77/78 are interconnected thereto for the collection of electron current 
respectively. 
Theoretically, the operation of such oxygen sensors occur by O.sub.2 
molecules becoming oxygen ions with the addition of four electrons at the 
surface of electrode 74. The oxygen ions then diffuse into the solid 
electrolyte 73. Since the partial pressure of oxygen is higher on surface 
74 than on surface 76, the net oxygen ions will move freely through the 
solid electrolyte to the outer catalytic electrode 75. At this point, the 
oxygen ions will give up electrons and combine to form O.sub.2 molecules 
once more. A net voltage will thus develop between electrode 74 and 
electrode 75 in response to the difference of partial pressure of O.sub.2 
between the exhaust gas and the ambient atmosphere. Increasing the 
difference in partial pressures between the electrodes will, as a rule, 
increase the voltage created. Generally, a net partial pressure of O.sub.2 
in the exhaust gas of about 10.sup.-22 atmospheres (corresponding to rich 
A/F mixtures) will cause the sensor to output a voltage in the order of 
1.0 volts. When the net pressure of oxygen increases, the sensor output 
voltage decreases, becoming less than 0.1-0.2 volts when the new partial 
pressure of O.sub.2 in the exhaust gas is 10.sup.-2 atmospheres or more 
(corresponding to lean A/F mixtures). 
It has been discovered that a sensor without the thin platinum overlayer 
75(a), equivalent to the structure in FIG. 16(b), cannot equilibrate the 
exhaust gas behind a bad three-way catalyst; the sensor is essentially a 
low-to-noncatalytic electrode sensor. Such construction merely has an 
outer platinum electrode 99 in the form of a long, thick platinum strip 
but absent a thin porous platinum overlayer. It is the platinum overlayer 
that promotes the high catalytic activity. 
An integrated closed-loop sensor construction is shown in FIG. 17 as an 
improved and alternative embodiment. Using two separate EGO sensors to 
implement the method of this invention, differences might arise due to 
sensor location, local flow, different heater temperature, etc. These 
differences could be minimized by carefully engineering, but are largely 
eliminated by constructing the two sensors on a single substrate as shown 
in FIG. 17. This will enhance the accuracy of the monitoring over that 
performed using two sensors. The proposed device of FIG. 17 incorporates 
two oxygen sensors on a single oxygen conductive substrate 80. Both 
devices would then be subjected to nearly identical location, flow, 
temperature, and aging conditions to lower output differences because of 
these interfering factors. The electrolyte is separated into two portions 
80(a) and 80(b) by a material to prevent cross-talk (O.sub.2 transfer 
between two sensors). Alumina (Al.sub.2 O.sub.3) may be used as such 
insulating material 87. The portion of the sensor which would carry out 
highly catalyzed equilibration has an electrode 81 formed of a thin 
platinum strip accompanied by a highly catalytic overcoating of the 
electrolyte. To complete one part of the dual sensor, an electrode 83 is 
formed either of highly catalytic or noncatalytic material subjected to 
the air reference interior of the construction. On the other side of the 
construction is a low-to-noncatalyzed electrode 82 which is exposed to the 
exhaust gases and accompanied by its other electrode 84 subjected to the 
air reference side, which electrode may be either catalyzed or 
noncatalyzed. To improve the accuracy of the device, a heating element 85 
may be disposed to maintain the air reference temperature at an elevated 
level. Leads are connected to each of the electrodes as shown in FIG. 17 
and respectively are labeled G1, V1, G2, and V2. Both air reference side 
electrodes could be of identical material. Both devices would be mounted 
inside a housing containing a heater, although this might not be essential 
in the case of devices used for catalyst monitoring only. 
The insulating layer 87 (Al.sub.2 O.sub.3) can be eliminated if the 
alternative construction of FIG. 19 is utilized. Electrodes 83 and 84 (of 
FIG. 17) are formed as a common inner electrode 88. The two differentially 
catalyzed electrodes (81, 82 of FIG. 17) are shifted laterally to become 
electrodes 86 89; cross-talk is minimized. This construction is desirable 
for mechanical reasons because Al.sub.2 O.sub.3 can have a different 
thermal expansion coefficient than ZrO.sub.2 (which can result in 
cracking). 
Still another alternative embodiment within the concept of this invention 
is a simplified open-loop type sensor 90 (as shown in FIG. 18) that 
eliminates the air reference and provides a differential measurement of 
the exhaust gas between a highly catalytic electrode 93 and a 
low-to-noncatalytic electrode 92. It consists of a single block or piece 
91 of ZrO.sub.2 electrolyte having one side 94 adapted to receive a highly 
catalyzed electrode by utilizing the conventional platinum strip/platinum 
overlayer combination, the overlayer providing the porous film that is 
necessary to provide the high catalytic activity. The opposite side 95 
contains only a narrow, solid strip of platinum which operates 
low-to-noncatalytically. Alternatively, a thin silver layer may displace 
the solid platinum strip to operate as a noncatalytic electrode. The 
device is immersed completely in exhaust gases which makes the whole 
structure simpler and less expensive than a conventional EGO sensor. A 
central bored hole 97 completes the ability to immerse the entire 
electrolyte in exhaust gas. This type of sensor functions because the 
partial pressure between an equilibrated gas and a nonequilibrated gas 
promotes a difference in voltage. When the main catalyst is not 
functioning properly, this difference in voltage will be readily apparent 
However, when the main catalyst is operating properly, the difference in 
voltage will be relatively minor. Since this type of sensor will not 
operate as a switch type about stoichiometry, it cannot be used in a 
closed-loop fuel control system in addition to acting as a catalyst 
monitor.