CA1060097A

CA1060097A - Method and device for detection of fluctuation in air/fuel ratio of air-fuel mixture fed to internal combustion engine

Info

Publication number: CA1060097A
Application number: CA267,007A
Authority: CA
Inventors: Takeshi Fujishiro; Toru Kita
Original assignee: Nissan Motor Co Ltd
Current assignee: Nissan Motor Co Ltd
Priority date: 1975-12-03
Filing date: 1976-12-02
Publication date: 1979-08-07
Also published as: DE2654892C3; DE2654892B2; JPS5267697A; JPS5415517B2; DE2654892A1; GB1522720A

Abstract

Abstract of the Disclosure A sensor comprises a layer of an oxygen ion con-ductive solid electrolyte such as stabilized zirconia, two electrode layers of a catalytic metal such as platinum formed porously on both sides of the electro-lyte, only one of them formed on first side of the electrolyte layer being directly exposable to an exhaust gas stream, and a gas passage arranged to allow the exhaust gas to contact the other side of the electro-lyte layer with a time lag behind the contact of the gas stream with the first side. A fluctuation in the air/fuel ratio of an air-fuel mixture fed to the engine across the stoichiometric ratio can be detected by this sensor since the fluctuation and the aforementioned time lag cause a difference between oxygen partial pressure on one side of the electrolyte layer and that on the other side, resulting in development of an output voltage of the sensor.

Description

~060097 This invelltion re]ates to an improvement on a method of detecting the air/fuel ratio of an air-fuel mixture consumed in an internal combustion engine by examining the concentration of oxygen in the exhaust gas and a sensor which operates on the p~inciple of solid clectrolyte oxygen concentration cell but can produce an electrical signal clearly indicating a variation of the air/fuel ratio across a stoichiometric ratio.
10 A conventional oxygen sensor which operates the principle of concentration cell has a layer of solid electrolyte in which oxygen ions work as carriers and two electron conductive electrode layers which are porously formed on both sides of the electrolyte layer.
A typical example of the solid electrolyte is zirconia ceramic containing a stabilizing component such as calcia, and the electrode layers are usually made of platinum. The oxygen concentration in a gas, for exa-mple exhaust gas of an internal combustion engine, is examined by exposing the two electrode layers of this sensor respectively to the gas subject to examination and a reference gas such as air. Then the sensor develops an electromotive force across the two electrodes accord-ing to the difference in oxygen partial pressure between the examined gas and the reference gas. This electromotive '::

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~060097 force l~ is determille~l by the ~ernst's equation:

E = -r~ loge P = ~ 1~10 P~ .................... (1) where R is the gas constant, T reprcsents the absolute temperature, F is the Faraday constant, K is a constant, P represents oxygen partial pressure, and the subscripts 1 and 2 refer to the gas subject to measurement and the reference gas, respectively. Thus the electromoti~e force E or output voltage of this sensor is in dependence on temperature, so that the electrolyte layer of the ffensor should be kept at an elevated temperature in practical operation.
In the exhast gas of an internal combustion engine which is operated with a hydrocarbon fuel typified by gasoline, carbon monoxide and unburned hydrocarbons react with oxygen remaining in the exhaust gas. Accord-ingly the following reactions are considered to re- ~
;~ speetively be in equilibrium states in the exhaust gas.

CO + 2 2 2 H2 + 2 2 = H20 These equilibrium states and, hence, the oxygen partial ~::

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pressure in the exhalist gas are in dependence on the exhaust gas temperature. Platinum is used as the material of the electrode layers of the sensor partly because of its catalytic ability on these oxidation reactions. Upon contact of the exhaust gas with the platinum electrode, these two reactions rapidly proceed towards the right side. Because of the occurrence of such oxidation reactions on one side of the solid eLe-ctrolyte layer and a great dependency of the amount of oxygen in the exhaust gas on the air/fuel ratio of the air-fuel mixture consumed in the engine, the described oxygen sensor exhibits the following OUtpllt charac-teristic. When the air/fuel ratio is represented by excessairfactor ~ which can be defined as the ratio f the air-to-fuel ratio of an air-fuel mixture con-sumed in the engine to the stoichiometric air-to-fuel ratio of the same components, the output voltage of the sensor stands at a rela*ively high level, only slightly affected by a variation in the air-fuel ratio, (assuming that the exhaust gas temperature does not substantially vary) so long as the value of ~ is smaller .
than 1.0 but stands at a distinctly lower level while is larger than 1Ø If the value of ~ varies across 1.0, the output voltage exhibits an abrupt transition from one of these two levels to the other.

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As is wcll known, an o~ygel~ sensor of the de-scribed type is suitab:le for use as a detection ele~nent in a feedback control system for controllillg the air-fuel ratio of an air-fuel mixture fed to arl internal combustion engine, particularly, for automotive use in connection with the prevention of air pollution and~or improvement on the fuel economy. The use of this oxygen sensor is especially advantageous when the control system aims at maintaining the excess air factor at or in the viclnity of 1.0 as does in many cases.
In practical applications, the solid electrolyte layer of this oxygen sensor in most cases is formed into the shape of a tube which is closed at one end for con-venience of attachment to1 for example, an exhaust pipe for the engine and exposure of only one(outer) side of the electrolyte layer to the exhaust gas.
However, conventional oxygen sensors of the de-scribed type have the following disadvantages.
(1) A hermetic and heat-resistant seal is in-dispensable to the sensors for completely isolating one side of the electrolyte layér from the exhaust ga~
(2) The sensors become inoperable when the solid electrolyte layer, which is not very tough, cracks due - ;~
to, for example, thermal shocks.
. 25 ~3) The solid el-ctro y e layer cannot readily ~; ~

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~060097 and uniformly be heated t;o a desirable temperatuIe since the electrolyte layer is heated only from one side.
It is an object of the present invention to pro-vide an improved method of detecting the air/fuel ratio of an air-fuel mixture red to an internal combustion engine, whicll method utilizes a sensor op~rating on the principle of solid electrolyte oxygen concentration cell without involving the above described disadvantages of conventional oxygen sensors.
It is another object of the invention to provide an improved method of detecting the occurrence of a variation in the air/fuel ratio of an air-fuel mixture fed to an internal combustion engine across the stoi-chiometric air/fuel ratio.
It is still another object of the invention to provide a sensor which operates on the principle of solid electrolyte oxygen concentration cell and is free from the above described disadvantages of conventional oxygen sensors.
~20 It i8 still another object of the invention to provide a sensor which can produce, when exposed to exhaust gas of an internal combustion engine, an ele-ctrical signal clearly indicating a fluctuation in the air/fuel ratio of an air-fuel mixture consumed in the engine across the stoichiometric air/fuel ratio.
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: ' ' .: - . .: ,, .. , An air/fue]. ratio detection method according to the invention utilizes a sensor having a layer of an oxygen ion conductive solid electrolyte and two porous electrode layers of a metal having catalytic ability on oxidation reactions of carbon monoxide and hydrocarbons formed respecti-vely on both sides of the electrolyte layer. The method com-prises the steps of contacting a stream of the exhaust gas of an internal combustion engine with first one side of the electro-lyte layer of the sensor through the electrode layer formed thereon, and then contacting the same exhaust gas with the other (second) side of the electrolyte layer with a time lag behind the contact of the exhaust gas stream with the first side of the electrolyte layer, so that the sensor produces an .-output voltage when the air/fuel ratio fluctuates across the ~:
stoichiometric ratio and a difference arises between oxygen partial pressure on one side of the electrolyte layer and that on the other side due to both a fluctuation in the oxygen .
concentration in the exhaust gas stream and the aforementioned time lag. . :-:~ .
A sensor according to the present invention comprises:
a tubular body having a closed end, at least a portion of the wall of the body including the closed end being an oxygen ion conductive solid electrolyte layer; a pair of electrodes for ::
measurement of an electromotive force developed by the solid : ~ electrolyte, the electrodes consisting of a porous and electron conductive first electrode layer of a metal, which :.:
~catalysts oxidation reactions of carbon monoxide and hydro-~: carbons contained in the exhaust gas, formed on and in intimate contact with.the outer side of the solid electrolyte layer and a.porous and electron conductive second electrode layer of the metal formed on and in intimate contact with the inner side of the solid electrolyte layer; and a gas passage .

in the form of at least two holes bored through the wall of the body at locations relatively close to the closed end for allowing the exhaust gas to pass through the interior of the body when an end portion including the closed end of the body is disposed in a stream of the exhaust gas, each of the at least two holes having a cross-sectional area small enough to offer a substantial resistance to the inflow of the exhaust gas therethrough into the interior of the body so that the exhaust gas comes into contact with the inner side of the solid electrolyte layer with a time lag behind the contact of the exhaust gas stream with the outer side of the solid electro-lyte layer.
The air/fuel ratio detection method according to the invention is particularly suitable to an engine system in which the air/fuel ratio is intended to be maintained at or in the vicinity of the stoichiometric - : : : , - . . . . , "

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, ` . ' , ' ; ~ ' ' . , . . ', ' '~ ` ... ~ , ',,,, ' airJfuel ratio alld l~as all a(lvantage t}~nt~ ~hen the air/fuel ratio fluc1,uates acro.ss the stoichiometric ratio, the direction of the fluctuation, that is, whether the fluctuati,on is from the lower side -to tlle higher side or contrary, can clearly be identifie,d from the polarity of the output voltage of the sensor:
the output voltage is a negative one in the former case but positive in the latter case.
The sensor according to the invention has the l~ advantages, besides the described output characteristic, that the sensor can operate even when cracks are present in the electrolyte layer and that the electrolyte layer is heated from both sides and accordingly can readily be heated even under an unfavorable condition as, for example, in cold starting of the engine.
The invention will fully be understood from the following detailed description of preferred embodiments ,' with reference to the accompanying drawings, wherein:
Fig. l is a longitudinal sectional view of a sensor as a flrst embodiment of the invention;
Fig. 2 is a fragmentary and sectional view showing ,~ a minor modification of the sensor of Fig. l;
Fig. 3 is a longituain,al sectional view of a con-ventional oxygen sen 90r; .-Fig. 4 is a longitudinal sectional view of a sensor _ 9 _ .. ..
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~060097 as another embodime~t of tlle invelltion;
~ig. 5 is a fragmentary and sectional ~iew showing a minor modification of the sellsor of Fig. 4;
Fig. 6 is a graph showing the relationship between the air/fuel ratio of an air-fuel mixture consumed in an internal combustion engine and the oxygen partial pressure in the exhaust gas of the engine;
Fig. 7 presents a pair of charts showing a manner of variation in thè oxygen partial pressure occurring when the air/fuel ratio exhibits a periodical fluctu ~ ~ -ation around a stoichiometric ratio;
Fig. 8 presents a pair of charts showing a time lag in the development of an oxygen partial pressure on the inside of a solid electrolyte layer in a sensor ~ -according to the invention behind the development of the same oxygen partial pressure on the outside of the ;~
same layer;
Fig. 9 is a chart showing a periodical variation in the magnitude of the difference between the oxygen ~; 20 partial pressures on the two sides of the same electro-lyte layer derived from the charts of Figs. 7 and 8;
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Fig. 10 is a chart showing a periodical fluctuation -. in the output voltage of the sensor resulting from the variation shown In Fig. 9; and Fig. 11 is a chart showing the same as Fig. 10 - ' ~
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~ 1060097 but in a form lllore faithful -to an actual function of the sensor in a practical engine system.
A sensor 10 of Fig. 1 as an embodiment of the in-vention operates on the known principle of an oxygen concentration cell and has a layer in the form of a tube 12 of an oxygen ion conductive solid electrolyte typified by a zirconia ceramic containing calcia as a stabilizing component. The solid electrolyte tube 12 is closed at one end. The outer surface of this tube 12 is entirely coated with a porous and electron con-ductive electrode layer 14. This electrode layer 14 is made of a metal such as platinum which has a catalytic activity on the oxidation of oxidizable components of the exhaust gas of an internal combustion engine. The inner surface of the solid electrolyte tube 12 is en-tirely coated with an electrode layer 16 which is similar to the outer electrode layer 14 both in the material and in the structure.
The outer diameter of the solid electrolyte tube 12 is locally enlarged at a middle section to form an -annular ridge or collar 12a. The tube 12 is inserted ~ into a tubular metal shell 18 the inner diameter of `~ whlch is locally enlarged to fit with the collar i2a of the tube 12 such that a closed end portion of the tube 12 protrudes from the metal shell 18. This l~ta~

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This metal. shell 1~ has on its outside a fixture means such as threads to airti.ghtly insert the ~rotruded portion of the electrolyte tube 12 into an exhaust pipe 20 for an internal combustion engine, for example, through a boss 22 formed on the wall of the exhaust pipe 20. I`he metal shell 18 serves also as a conductor for the outer electrode layer 14. The solid electrolyte tube 12 and the metal shell 18 are so shaped as to pro-vide an annular space therebetween at a region fro~ the right side end of the collar 12a to the open end of the electrolyte tube 12. This annular space is fil~ed with ~.
a powdery and electrically conductive sealing agent 24 such as, for example, graphite powder, copper powder or semiconductive CuO powder, or a powdery mixture of such .
a conductive material and a nonconductive and refractory material, which has been compacted in the annular space.
To support the compacted sealing agent 24 and fix the electrolyte tube 12 to the metal shell 18, rings 26 of a metal such as copper are forcibly inserted i.nto the annular space. These rings 26 contribute also to the assurance of electrical connection between the outer electrode layer 14 and the metal.shell 18. To further assure the fixing of the electrolyte tube 12 to the metal shell 18, a tubular metal retainer 28 is tightly 25. inserted to an open end (right side end) portion of the . . annular space by means o~ threads.

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An open end port:ion Or tile bore of the e]ectrolyte tube 12 is made to have ao elllarged diameter with a tapered section, and a meta] conductor n1ember 30 which takes the form Or a solid cylinder with a tapered flange at one end is partly inserted into the bore of the electrolyte tube 12 such that the tapered f]ange fits with the tapered section of the bore. The conductor member 30 has such an outer diameter that an annular space is formed between the non-flanged portion of 10. this member 30 and the inner electrode layer 16. This annular space is filled with a powdery and electrically conductive sealing agent 32, which has been compacted and may be of the same material as the sealing agent 24.
Metal rings 34 are forcibly inserted into the annular space around the conductor member 30 to support the sealing agent 32 and assure the electrical connection between the inner electrode layer 16 and the conductor member 30.
As an essential feature of a sensor according to the invention, one or a plurality of holes 36 are formed in and through the wall of the solid electrolyte tube 12 and the two electrode layers 14 and 16 at a closed ~: end region protruding from the metal shell 18. These holes 36 are formed usually but not necessarily radially of the tube 12. As the result, the exhaust gas passing : , ' , -s' :
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106l)097 through the exllaust pipe 22 can enter the illterior Or the electrolyte tube 12. I-lowever, the cross-sectional area of the holes 36 are sl1lall enough to offer a certain resistance to the inflow o~ the exhaust gas into the interior of the tube 12~ Accordingly, the exhaust gas arrives on the inner surface of the electrolyte tube 12 with a certain time lag behind its arrival on the outer surface of the electrolyte tube 12 (both the outer and inner electrode layers 1ll and 16 are porous and gas permeable). The interior of the electrolyte tube 12 in the sensor of Fig. 1 is isolated from the atmosphere by the conductor member 30, so that the amount of the aforementioned time lag is determined by the cross-sectional area of the holes 36 and the volume of the interior space defined in the electrolyte tube 12.
Fig. 2 shows another sensor 50 as a slight modifi-cation of the sensor 10 of Fig. 1. In this case, the conductor member 30 has an axial hole 38 in order to pASS therethrough the exhaust gas admitted into the interior of the electrolyte tube 12 into an exterior environment, for example an air cleaner for the engine, which is substantially under the atmospheric pressure.
In other respects the sensor 50 of Fig. 2 has the same ; construction as the sensor 10 of Fig. 1. In this case .. . .
~ the amount of the time lag in the arrival of the exhaust ' ' . . . .
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',, , ' J ~ .'' gas on the inside of the electrolyte tube 12 behind the arrival on the outside depends on the differeMce between the exhaust pressure and the atmospheric pres-sure in addition to the cross-sectional area of the holes 36.
A conventional oxygen sensor 60 which is con~
structed essentially similarly to the sensor 50 of Fig. 2 is shown in ~ig. 3. As a sole difference from the sensor 50 of ~ig. 2, this conventional oxygen sensor 60 has no holes in the wall of the electrolyte tube 12, because it is necessary for this type of oxygen sensor 60 that the inside of the electrolyte tube 12 is com-pletely isolated from the exhaust gas and is exposed to atmospheric air as a reference gas.
The sensors 10 and 50 are not suitable for exactly measuring the oxygen concentration in the exhaust gas but are quite suitable for examining a fIuctuation in the air/fuel ratio of a combustible mixture fed to the engine across the stoichiometric ratio as will be understood from the following explanation of the function of these sensors 10 and 50 in the exhaust gas.
When an exhaust gas resulting from the combustion of an air-gasoline mixture in an internal combustion engine is contacted with a catalytic metal such as platinum at a high temperature, an equilibrium partial : ~:

pressllre P(atm) of oxygen in the exhaust gas varies Wit]l variation~ in the air/fuel ratio, i.e. excess air factor ~ , of the mixture and the exhaust gas temperature in a manner as shown in Fig. 6. The oxygen partial pressure P stands at two distinct]y diff,erent levels according as the value of ~ remains on either side of 1.0 where the air/fue] ratio is stoichiomctric.
At 600 C, for example, the oxygen partial pressure P is on the order of iO 20 while-the value of 7~ is smaller than 1.0 but on the order of 10-2 while ~ is larger than 1Ø The oxygen partial pressure P exhibits an abrupt transition from one of these two levels to the '' other when ~ varies across 1Ø
The operation of a feedback system for maintaining the value of ~ at 1.0 will result in a small magnitude of periodical fluctuation of the value of ~ around and across 1.0 as typified by the chart (A) of Fig. 7. On the basis of this chart (A) and the graph of Fig. 6, the oxygen partial pressure P in the exhaust gas (upon ~' contact with platinum) at 600 C exhibits a perlodical , fluctuation in a manner as shown by the chart (B) of Fig. 7. ,-' ` If the sensor 10 or 50 according to the invention is exposed to a stream of the exhaust gas in which the oxygen partial pressure P fluctuates as shown in Fig. 7, ' ":'; :' ~ -.
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the fl.uctllatin~ ~xygen partia]. pressure P is ap~.lied ~nto the outside of the electrolyte tube 12 since tlle exhau.st gas can pass tl-roug~l the outer electrode layer 14. ~or the outside of the electrolyte tube 12, thi.s oxygen partial pressure P will hereil-lafter be represented by Pl for convenience in explanation. In Fig. 8, the chart (I), which is substantially identical with the chart (B) of Fig. 7, shows the fluctuation in the oxygen partial pressure P1 in the exhaust gas stream on the outside of the electro]yte tube 12. Then the exhaust gas enters the interior of the electrolyte tube 12 through the holes 36 and arrives on the inside of the electrolyte tube 12 through the inner electrode layer 16 with a time lag behind the arrival of the same exhaust gas on the outside of the tube 12. For the inside of the tube 12, the oxygen partial pressure will ~:
hereinafter be represented by P2. Since the oxygen partial pressure P or P1 exhibits a periodical fluctu- :
ation as shown in the chart (I), P2 exhibits a fluctu- . ~
ation fundamentally in the same manner. However, there ...
lS a phase difference between the fluctuation of Pl and that of P2 due to the aforementioned time lag. .
In contrast to the chart (I), the periodical fluctuation of P2~is shown by the chart (II) in Fig. 8. The magnitude 2~ of the phase difference between the curve of the chart .

(I) aI~d that of tlle clllrt (TI) OI' the .lmoullt of tlle time ]~g ~et~een the ~rrival o~ the exl~clust gas Oll the outside and illside of the electrolyte tul)e 12 is in-dicated at d j.M Fig. 8.
When tlle amount of -the time lag d (wllich depends on the desigrl of the holes 36 of` the sensor 10 or 50) is appropriately preset in relation to the frequency of the periodical fluctuation in the oxygen partial pressure P or fluctuation in the value of ~, there arises a difference between the magnitudes of Pl and P2 intermittently for certain periods of` time as seen in Fig. 8. The EMF or O~ltput voltage E of the sen~or 10 or 50 is given by the following equation:

P , ': ' ,."
E = _~F_lOglo p2 " ' -= _7nF--(logl0p2 - 1gl0Pl) ..... (2 ( glOP2 loglOPl) calculated from the charts of Fig. 8 exhibits a periodical fluctuation as shown in Fig. 9.
Fig.10 shows a similar fluctuation of the output voltage E(volts) a~ the result of a numerical calculation ~ :
on the equation (2) ba~ed on the chart of Fig. 9. As `25 seen in Fig. 10, the output voltage E of the sensor 10 ~: .

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` 1060097 or 50 tal~es a vallle of about -] volt wl~erl the v~l~le of varies across 1.0 from a s~aller rarlge (~ C 1.0, meaning the presence of excess fuel in the air-fuel mixture) to a larger range (~ ~ 1.0, l~eallirlg shortage of fuel in the air-fuel mixture) but another value of about +l volt when ~ varies across 1.0 from a larger range to a smaller range. The output voltage E re~ains substantial.ly at zero volt while the value of ~ remains either above or below 1Ø The output voltage E varies : .

depending on the exhaust gas temperature as demonstrated in Fig. 6, but exhibits a periodical fluctuation in a manner as shown in Fig. 10 regardles of the exhaust gas temperature so long as the value of i~ varies across -.. :
1Ø It will be understood that the waveform in Fig. -::: :
10 is an idealized one and that an actual waveform of the output voltage E in practical use of the sensor 10 - :~
or 50 is somewhat deformed and/or rippled as shown in Fig. ll because of minute and continual variations in various factors including the value of ~ . .

;20 In a sensor according to the invention, the solid elec~rolyte layer does not necessarily take a tubular form as in Figs. 1 and 2. ..
Referring to Fig. 4, a sensor 70 as another embodi-ment of the invention has an oxygen ion conductive ~: 25 solid electrolyte layer 12A which takes the form of a .

19 _ : -: . .
. ' : ' disk and is fixedly a~ld airt,iglltly rcceived in a tubul~r meta~ sllell l~A t,o close one end of tlle shell l~A. Tle outer and inner electrode layers 14 and 16 are formed on both sides of the electro],yte disk 12A f~ damentally in the same manner as in the sensor 10 of Fig. 1. The outer electrode layer 14 is in contact with t]le meta],' shell loA, but the inner electrode layer 16 is isolated from the metal shell loA. The other end of the tubular shell loA is airtightly closed by a disk 40 of a heat-resistant and electrically insulating material such asceramics. A conductor member 42 is passed through and airtightly fixed to the closure disk 40, and a lead 44 of, for example, platinum wire connects the inner ele- ' ctrode layer 16 with the conductor member 42 through the bore of the shell 18A. The shell 18A has threads on its outside and a flange or collar at one end region remoter from the electrolyte disk 12A, so that a portion of the shell 18A including one end closed by , the electrolyte disk 12A can be inserted into the ex-haust pipe. In this portion, o-ne or a plurality of radial holes 46 are formed in the wall of the shell loA ~ '~
' to serve as a gas passage for admitting the exhaust gas ; into the interior of the shell 18A with a certain ~- ' resistance. It will be apparent that the sensor 70 operate~ in the same manner as the sensor 10 of Fig. 1.
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Su~ta~ltially silnilarly to thc mo~ificatioll Or the sel~sor 10 o~ l~`ig. 1 to the sensor 50 of Fig. 2, tlle metal s]-ell 18A of the sen.sor 70 may have a gas outlet hole 1l~, as showll in Fig. 5, at; a loca1;ion remaining outside of tlle exllaus~ pipe.
A sensor accord:ing to the invention, regardless of its configuration, may comprise additional components such as, for example, porous protective coatings (not shown) on the surfaces of the outer and/or inner ele-1~ ctrode layers 14, 16 and a heat shield member aroulld the shell 18 or 18A as conventional oxygen sensors of the solid electrolyte concentration cell type do.
As will have been understood from the foregoing description, a sensor according to the invention is quite useful as an element of a feedback control system for maintaining the air/fuel ratio of an air-fuel mixture fed to an internal combustion engine exactly at or in the vicinity of a stoichiometric ratio. Compared with the use of a conventional oxygen sensor which is funda-mentally constructed as shown in Fig. 3, the method according to the invention has the following advantages.
(1) A sensor according to the invention can operate even if the electrolyte layer cracks due to, --for example, thermal shocks.
(2) The solid electrolyte layer can readily and - .

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uniformly l)e heated hy tl~e exhaust gas.

(3) The requirement for the hermetic seal is lessned particlllarly wllen the in:side of the electrolyte layer is isolated from the atmosphere. Besides, it is not necessary to provide any condllit for exposing the inside of the electrolyte layer to air.
(1~) It can easily and doubtless be identified - whether the amount of fuel in the air-fuel mixture has varied from excess to shortage or contrary since the lQ polarity of the output voltage of the sensor varies depending on the direction of the transition of the air/fuel ratio across the stoichiometric ratio.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A method of detecting the occurrence of a fluctuation in the air/fuel ratio of an air-fuel mixture fed to an internal combustion engine across the stoi-chiometric air/fuel ratio, the method comprising the steps of:

disposing a sensor in a passage of a stream of the exhaust gas of the engine, said sensor having a layer of an oxygen ion conductive solid electrolyte and porous and electron conductive first and second electrode layers of a metal having catalytic ability on oxidation reactions of carbon monoxide and hydro-carbons contained in the exhaust gas formed respectively on first and second sides of the electrolyte layer;

contacting said stream of the exhaust gas with the first side of said electrolyte layer of said sensor through said first electrode layer; and contacting the exhaust gas with the second side of said electrolyte layer with a time lag behind the contact of the exhaust gas with said first side, so that the sensor produces an output voltage when said air/fuel ratio fluctuates across the stoichiometric ratio and a difference arises between oxygen partial pressure on said first side and oxygen partial pressure on said second side due to both a fluctuation in the oxygen concentration in the exhaust gas and said time lag.

2. A method as claimed in Claim 1, wherein the last step is performed by shaping said sensor into the form of a tube an end of which is closed by at least a portion of said electrolyte layer with said first side outside and forming a gas passage through the wall of said tube to have a cross-sectional area so small as to offer a substantial resistance to the inflow of the exhaust gas into the interior of said tube.

3. A method as claimed in claim 2, wherein the exhaust gas admitted into the interior of said tube is allowed to flow out of said interior exclusively through said gas passage.

4. A method as claimed in claim 2, further com-prising the step of passing the exhaust gas admitted into the interior of said tube at least partially to an external environment which is substantially at the atmospheric pressure.

5. A method as claimed in claim 1, further com-prising the step of identifying whether said fluctuation in the air/fuel ratio across the stoichiometric ratio is a rising fluctuation or contrary based on the fact that said output voltage is a negative one in the case of said rising fluctuation but is a positive one in the case of a lowering fluctuation.

6. A sensor for detecting the occurrence of a fluctuation across the stoichiometric air/fuel ratio in the air/fuel ratio of an air-fuel mixture fed to an internal com-bustion engine by disposing the sensor into a stream of the exhaust gas of the engine, the sensor comprising:
a tubular body having a closed end, at least a portion of the wall of said body including said closed end being an oxygen ion conductive solid electrolyte layer;
a pair of electrodes for measurement of an electro-motive force developed by said solid electrolyte, the electro-des consisting of a porous and electron conductive first electrode layer of a metal, which catalysts oxidation reactions of carbon monoxide and hydrocarbons contained in the exhaust gas, formed on and in intimate contact with the outer side of said solid electrolyte layer and a porous and electron conductive second electrode layer of said metal formed on and in intimate contact with the inner side of said solid electrolyte layer; and a gas passage in the form of at least two holes bored through the wall of said body at locations relatively close to said closed end for allowing the exhaust gas to pass through the interior of said body when an end portion including said closed end of said body is disposed in a stream of the exhaust gas, each of said at least two holes having a cross-sectional area small enough to offer a substantial resistance to the inflow of the exhaust gas therethrough into the interior of said body so that the exhaust gas comes into contact with said inner side of said solid electrolyte layer with a time lag behind the contact of the exhaust gas stream with said outer side of said solid electrolyte layer.

7. A sensor as claimed in claim 6, wherein said tubular body is entirely said solid electrolyte layer.

8. A sensor as claimed in claim 7, further com-prising a tubular metal shell which receives therein said tubular body fixedly and airtightly such that an end of said shell is closed by said end portion of said body and that said shell is in contact with said first electrode layer.

9. A sensor as claimed in claim 7 or 8, wherein said tubular body has an open end, the sensor further comprising a closure member of a metal fixedly and airtightly inserted into an open end portion of said tubular body so as to be in contact with said second electrode layer.

10. A sensor as claimed in claim 7 or 8, wherein said tubular body has an open end, the sensor further compri-sing a conductor member which is inserted fixedly into an open end portion of said tubular body so as to be in contact with said second electrode layer and has an aperture to allow at least a portion of the exhaust gas admitted into the interior of said body through said at least two holes to flow out of said body through said aperture.

11. A sensor as claimed in claim 6, wherein said solid electrolyte layer takes the form of a disc which closes an end of an open-ended tubular member of a different material.

12. A sensor as claimed in claim 11, wherein said tubular member is of a metal, the sensor further comprising a closure member of an electrically insulating material fixedly and airtightly covering an open end of said tubular member and a conductor member which is airtightly passed through said closure member and electrically connected to said second electrode layer.

13. A sensor as claimed in claim 12, wherein said closure member is made of ceramics.

14. A sensor as claimed in claim 13, wherein said tubular member has an aperture formed in the wall thereof at a location close to said closure member to allow at least a portion of the exhaust gas admitted into said body to flow out of said body through said aperture.

15. A sensor as claimed in claim 6, wherein solid electrolyte is zirconia containing calcia as a stabilizing compo-nent.

16. A sensor as claimed in claim 15, wherein said first and second electrode layers are made of platinum.