Gas component detection apparatus

A gas component detection apparatus for detecting gas components contained in an exhaust gas from a combustion device and determining an air-fuel ratio of an air-fuel mixture supplied thereto. A gas component detecting element composed of a semiconductive metal oxide is contacted by the exhaust gas andexhibits variable electric resistances according to the concentrations of the gas components. The electric resistance is taken out as an electric signal through two electrodes disposed either on the surface of the detecting element exposed to the exhaust gas or within the detecting element adjacent that exposed surface. A porous coating layer having an exhaust gas permeability covers the surface of the detecting element exposed to the exhaust gas so as to prevent poisonous substances contained in the exhaust gas from depositing on that exposed surface of the detecting element. The coating layer is formed of a metal oxide having electrically insulating properties and carries therein a catalyst promoting an oxidation reaction of the exhaust gas. The detecting element exhibits an abruptly changed electric resistance when the actual air-fuel ratio of the air-fuel mixture supplied into the combustion device is deviated from a stoichiometrical air-fuel ratio, thereby enabling the control of the air-fuel ratio of the air-fuel mixture.

BACKGROUND OF THE INVENTION 
1. FIELD OF THE INVENTION 
The present invention relates to a gas component detection apparatus for 
detecting with high sensitivity and rapid response variations in 
concentrations of gas components such as oxygen (O.sub.2), carbon monoxide 
(CO) and hydrocarbon (HC) contained in a gaseous mixture. The invention 
finds its useful application particularly in a gas component detection 
apparatus for an exhaust gas of an internal combustion engine. 
2. DESCRIPTION OF THE PRIOR ART 
Gas component detection apparatuses have been widely used in many 
industrial fields. Lately, as a counter measure to cope with the problem 
of an exhaust gas of internal combustion engine, the gas component 
detection apparatuses are employed for determining an air-fuel ratio of an 
air-fuel mixture supplied to an internal combustion engine. 
Heretofore, an electric cell comprising a solid electrolyte such as 
zirconia has been employed for the detection of the air-fuel ratio of 
air-fuel mixture supplied to the internal combustion engine. More 
particularly, the variation in the electromotive force of the cell 
depending on the concentration of oxygen contained in the exhaust gas was 
utilized as a measure for indicating a corresponding variation in the 
air-fuel ratio. Further, it has been also known to employ a semiconductive 
metal oxide and detect the variation in the electric resistance thereof as 
a measure to represent the variation in the air-fuel ratio. 
In the case of the solid electrolyte cell, the electromotive force is 
generated by the migration of ions through the lattice defects of the 
solid electrolyte under thermal excitation, so that when the temperature 
of the solid electrolyte is lower than 400.degree. C no electromotiveforce 
is generated. Accordingly, the gas component detection apparatus utilizing 
such solid electrolyte has a serious drawback such that the response of 
the apparatus is remarkably lowered when the temperature of the exhaust 
gas is low as is the case of starting the internal combustion engine. 
On the other hand, the detection apparatus utilizing the variation of 
electric resistance of a semiconductive metal oxide suffers from a 
disadvantage such that the detection of variation in the air-fuel ratio 
with respect to a predetermined value can not be effected with a desired 
accuracy since the characteristic curve representing the resistance 
variation of the semiconductive metal oxide relative to the variation in 
the air-fuel ratio has a relatively gentle inclination. Furthermore, when 
the gas component detection apparatus incorporating the semiconductive 
metal oxide as the detecting element is employed for determining the 
air-fuel ratio of the air-fuel mixture supplied to the engine, there 
arises the following problems. Namely, the surface of the semiconductor 
detecting element exposed to the exhaust gas during usage of the detection 
apparatus is deposited and gradually accumulated with poisonous substances 
such as phosphorus (P), lead (Pb), sulfur (S) and compounds thereof 
contained in the exhaust gas in addition to H.sub.2, CO, HC, O.sub.2, 
whereby the material constituting the detecting element will react with 
such poisonous substances to form compounds, incurring deterioration of 
the performance of the semiconductor detecting element. In such case, the 
characteristic curve of the detecting element is adversely changed. 
Further, when a catalyst is carried on the surface of the metal oxide 
constituting the detecting element, the performance or activity of the 
catalyst is also subjected to deterioration. 
SUMMARY OF THE INVENTION 
Accordingly, an object of the present invention is to provide a gas 
component detection apparatus for detecting gas components in an exhaust 
gas from a combustion device which can operate with high sensitivity and 
rapid response even when the temperature of the exhaust gas is relatively 
low. 
Another object of the invention is to provide a gas component detection 
apparatus in which the surface of a gas detecting element of a 
semiconductive metal oxide adapted to contact the exhaust gas is protected 
from being directly deposited with poisonous substances contained in the 
exhaust gas, thereby to stabilize the semiconductive characteristic of the 
gas component detecting element for a long use life. 
A further object of the invention is to provide a gas component detection 
apparatus in which a catalyst is protected from being degraded in its 
performance with the poisonous substances contained in the exhaust gas. 
In view of the above objects, according to one aspect of the present 
invention, there is provided a gas component detection apparatus which 
comprises a gas component detecting element composed of a thin film or a 
sintered mass of a semiconductive metal oxide, a catalyst composed of 
catalyst materials such as platinum and disposed on the side of the gas 
component detecting element exposed to the exhaust gas from a combustion 
device, and a porous coating layer having an exhaust gas permeability and 
formed of materials having a heat resistance. The porous coating layer is 
disposed at the side of the gas component detecting element exposed to the 
exhaust gas. Further, the detection apparatus is provided with two 
electrodes disposed either on the surface of the detecting element exposed 
to the exhaust gas or embedded in the detecting element adjacent that 
exposed surface in a manner to prevent formation of a short-circuit 
between the electrodes. 
According to another feature of the invention, there is provided a gas 
component detection apparatus for an exhaust gas from a combustion device 
such as an internal combustion engine in which the electric resistance of 
the gas component detecting element of a semiconductive metal oxide is 
caused to vary abruptly, when the detected air-fuel ratio is deviated from 
a stoichiometric air-fuel ratio serving as a reference value, thereby to 
enhance the detection accuracy of the apparatus. 
The above and other objects as well as novel features and advantages of the 
invention will become more apparent from the following description when 
read in conjunction with the accompany drawings.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Now, the invention will be described in detail in conjunction with 
exemplary embodiments thereof. On these embodiments, the gas component 
detection apparatus according to the invention is intended to be used as a 
means for determining the air-fuel ratio of the air-fuel mixture supplied 
to an internal combustion engine. In this connection, reference should 
first be made to FIG. 1A. The illustrated internal combustion engine 200 
having a carbureter 100 is provided with a three-component catalyst 
converter 300 with a view to eliminating three gas components, that is, 
CO, HC and NOx. In order to have a catalyst exhibit a maximum performance, 
it is required to maintain constantly the air-fuel ratio of the air-fuel 
mixture in a very narrow range W as is shown in FIG. 1B. However, in an 
ordinary internal combustion engine equipped with a conventional 
carbureter and fuel injection apparatus, the air-fuel ratio is actually 
inevitably subjected to a large variation, even when the ratio of an 
injected fuel to a sucked air is set to become constant. Accordingly, in 
order to maintain the constant air-fuel ratio, it is necessary to detect 
the actual air-fuel ratio and feed back the corresponding signal to the 
carbureter or the injection apparatus; thus controlling the air-fuel ratio 
of the supplied air-fuel mixture. 
To this end, the gas component detection apparatus 400 can be employed. The 
detection apparatus 400 detects the air-fuel ratio based on the fact that 
the variation in concentrations of gas components contained in the exhaust 
gas is closely related to the variation of the air-fuel ratio. In this 
connection, consideration has to be paid on the fact that the temperature 
of the exhaust gas as well as the concentrations of the gas components 
will vary abruptly and remarkably. Therefore, the gas component detection 
apparatus must be operated with high accuracy for a long use life 
notwithstanding of such prominent variables. The gas component apparatus 
according to the invention is constructed to adequately meet this 
requirement. 
Referring to FIGS. 2 to 5, reference numeral 1 denotes a housing of the gas 
component detection apparatus which is made of a metal material having 
high electric conductivity and adapted to be mounted on an exhaust 
manifold or the like portion of the internal combustion engine. To this 
end, the housing 1 has a threaded portion 1a and a tightening portion 1b. 
Accomodated at the lower portion within the housing 1 is a disk-like lower 
holding member 2 which is made of an electrically insulating material such 
as ceramic and formed with a tapered portion 2a along the lower peripheral 
edge thereof. As can be seen from FIGS. 2 and 3, the lower holding member 
2 is disposed in a counter-bore formed in the lower end portion of the 
housing 1 and securely held in place with a packing washer 3 interposed 
between the housing 1 and the top surface of the holding member 2, and a 
metal ring 4 of high electric conductivity interposed between the tapered 
portion 2a and the inwardly bent lower end portion of the housing 1 which 
is formed by heat-caulking. Disposed at the upper portion within the 
housing 1 is a column-like upper holding member 5 which is also made of an 
electric insulation material such as ceramic and formed with a tapered 
portion 5a along the upper edge portion thereof. The upper holding member 
5 is fixedly positioned in place by heat-caulking the upper edge portion 
of the housing 1 with a packing washer 6 and a metal ring 7 interposed 
between the holding member 5 and the inner wall of the housing 1 in a 
similar manner as the lower holding member 2. The lower and the upper 
holding members 2 and 5 are formed, respectively, with paired 
through-holes 2b; 2c and 5b; 5c extending axially. A first lead wire 8 is 
inserted into the through-holes 2b and 5b of the lower and the upper 
holding members 2 and 5. While a second lead wire 9 is inserted through 
the other holes 2c and 5c of the holding members 2 and 5. These lead wires 
8, 9 may be made of a heat-resistant metallic material having a high 
electric conductivity such as those commercially available under the name 
of SUS42 or INCONEL 600. 
A first thin film electrode 10 is disposed on the underside of the lower 
holding member 2 in alignment with the through-hole 2b, while a second 
electrode 11 also in a form of a thin film is mounted in alignment with 
the other through-hole 2c. These electrodes 10 and 11 may be made of a 
metal having an excellent heat and corrosion resistance such as gold (Au) 
and platinum (Pt) and deposited on the lower surface of the holding member 
2 by a suitable process such as vacuum evaporation or spattering. The 
first and the second electrodes 10 and 11 are connected to the associated 
lead wires 8 and 9, respectively, through an appropriate process such as 
heat-caulking, plasma welding or the like. 
Reference numeral 13 designates a gas component detecting element in a form 
of a thin film having a thickness preferably in the order of 300 A and 
formed of a transition metal oxide having a semiconductivity. The gas 
component detecting element 13 is coated with an insulation layer 14 (FIG. 
3) at the lower surface exposed to the exhaust gas. The insulation layer 
14 is made of a heat-resisting and porous metal oxide such as 
.gamma.-alumina (Al.sub.2 O.sub.3), zirconium dioxide (ZrO.sub.2), 
magnesium oxide (MgO) and has an exhaust gas permeability. It has been 
found that .gamma.-alumina is most preferable as the metal oxide for the 
insulation layer 14. In this case, the insulation layer 14 may be 
conveniently formed through a plasma-injection of .gamma.-Al.sub.2 O.sub.3 
or alternatively by applying a slurry containing .gamma.-Al.sub.2 O.sub.3 
together with a binding agent to the exposed lower surface of the gas 
component detecting element 13 and thereafter drying and sintering or 
firing the applied slurry. The insulation layer 14 carries a catalyst 15 
composed of catalyst substances such as platinum (Pt), palladium (Pd) and 
rhodium (Rh). The deposition of the catalyst 15 may be effected through 
for example a vacuum-evaporation process or a reduction process. In FIG. 
5, reference character 13a denotes particles of semiconductive metal oxide 
constituting the gas component detecting element 13, 14a, designates 
particles of metal oxide constituting the insulation layer 14 and the 
symbol 15a denotes particles of the catalyst 15. 
The gas component detecting element 13 is disposed at the lower surface of 
the lower holding member 2 between the first and the second electrodes 10 
and 11 and electrically connected to these electrodes. 
As the semiconductive metal oxide for the gas component detecting element 
13, there may be used titanium oxide (such as TiO.sub.2), nickel oxide 
(NiO), cobalt oxide (CoO), manganese oxide (MnO), zinc oxide (ZnO) and 
copper oxide (CuO) which are transition metal oxides, as well as tin oxide 
(such as (SnO.sub.2). The detecting element 13 may be formed of suitable 
one of these semiconductive metal oxides and deposited at the lower 
surface of the holding member 2 in thickness of 100 A to 10 .mu. through 
vacuum evaporation, electron beam evaporation or any other suitable 
depositing technique. 
Referring again to FIG. 2, it can be seen that the through-holes 5b and 5c 
formed in the upper holding member 5 are enlarged at the upper portions 
thereof so as to have a larger inner diameter than that of the first and 
the second lead wires 8 and 9. The annular recesses thus formed between 
the inner surfaces of the through-holes 5b, 5c and the lead-wires 8, 9 
receive a first and a second terminal members 16 and 17 formed with 
respective collars 16a and 17a at thin lower ends. The collars 16a and 
17a, and press-fitted rings 18 and 19 serve to securely hold the terminal 
members 16 and 17 in place within the respective through-holes 5b and 5c 
of the upper holding member 5. Further, respective sealing glass materials 
20 and 21 are filled in the spaces within the through holes 5b and 5c 
defined between the collar 16a and the ring 18, and the collar 17a and the 
ring 19, respectively. The first lead wire 8 is inserted into the first 
terminal member 16 and welded thereto at the upper end portion, while the 
second lead wire 9 is inserted into the second terminal member 17 and 
welded thereto at the upper end portion. Numeral 22 denotes a gasket. 
Next, the operation of the gas component detection apparatus of the 
aforementioned construction will be described. The gas component detection 
apparatus is mounted on an exhaust manifold of an internal combustion 
engine through the threaded and the tightening portions 1a, 1b of the 
housing 1 in such a manner that the gas component detecting element 13 is 
exposed to the exhaust gas. As is well known, the exhaust gas contains 
therein gas components such as O.sub.2, NOx, CO, HC, H.sub.2, CO.sub.2 and 
N.sub.2, and the content or concentration of each of these gas components 
will vary in dependence upon the air-fuel ratio of the air-fuel mixture 
under unburnt condition. The semiconductive metal oxide constituting the 
gas component detecting element 13 will be influenced or acted mainly by 
the concentrations or partial pressures of O.sub.2, CO, HC and H.sub.2 
gases contained in the exhaust gas, and exhibit variable electric 
resistance values according to the variation in the overall exhaust gas 
condition brought about by the variation in the partial pressures of these 
individual gas components. 
According to the invention, the insulation layer 14, which is disposed over 
the surface of the gas component detecting element 13 exposed to the 
exhaust gas, carries the catalyst 15 serving to enhance the reactivity of 
the element 13 to the gas components such as O.sub.2, CO, HC and H.sub.2. 
In particular, the catalyst 15 increases the sensitivity of the detecting 
element 13 to the variation in the partial pressure of O.sub.2 gas. 
Therefore, an abrupt and rapid change in the electric resistance value of 
the detecting element 13 can be produced when the variation in the partial 
pressure of O.sub.2 gas is occurred. This point will be further explained 
hereunder. 
When combustible gas components such as CO, HC and H.sub.2 are present 
along with O.sub.2 gas in the vicinity of the surface of the detecting 
element 13 exposed to the exhaust gas, the following reactions will take 
place under the action of the catalyst 15. 
EQU CO + 1/2 O.sub.2 .fwdarw. CO.sub.2 
EQU hc + 5/4 o.sub.2 .fwdarw. co.sub.2 + 1/2 h.sub.2 o 
EQU h.sub.2 + 1/2 o.sub.2 .fwdarw. h.sub.2 o 
when the air-fuel ratio of the air-fuel mixture is larger under unburnt 
state than the stoichiometrical air-fuel ratio, some amount of CO, HC and 
H.sub.2 gases will remain unreacted, even after all available O.sub.2 gas 
has been consumed for the reaction with CO, HC and H.sub.2. Accordingly, 
it is estimated that in this case almost no O.sub.2 gas will be present in 
the vicinity of the surface of the gas component detecting element 13. On 
the other hand, in case where the air-fuel ratio of the air-fuel mixture 
is smaller before combustion than the stoichiometrical air-fuel ratio, 
some amount of O.sub.2 gas will remain unreacted even after CO, HC and 
H.sub.2 gases have been all reacted with O.sub.2 gas. In this case, a 
certain quantity of O.sub.2 gas will exist near the surface of the gas 
component detecting element 13. It will thus be understood that there are 
produced near the surface of the detecting element 13 exposed to the 
exhaust gas two different states of atmosphere; i.e. the state in which a 
certain quantity of O.sub.2 gas is present and the state in which almost 
no O.sub.2 gas exists. Under the action of the catalyst 15 described, the 
electric resistance value of the gas component detecting element 13 will 
exhibit an abrupt varietion when the actual or detected air-fuel ratio is 
changed across the stoichiometrical air-fuel ratio. Thus, a voltage 
determined by the electric resistance value of the detecting element 13 
under the stoichiometrical air-fuel ratio can be used as a reference 
voltage, with which a voltage determined by the resistance value of the 
detecting element 13 under the detected air-fuel ratio is to be compared. 
More particularly, when the latter voltage is larger than the reference 
voltage, this means that the detected or actual air-fuel ratio is smaller 
than the stoichiometrical air-fuel ratio and vice-versa. Consequently, it 
becomes possible to control the actual air-fuel ratio so as to conform it 
with the stoichiometrical air-fuel ratio. 
FIG. 6 shows an example of an electric circuit which may be used to 
accomplish the control of the air-fuel ratio. In this circuit, the gas 
component detecting element 13 is represented by a detector resistor R, 
and a reference resistor R.sub.1 is connected in series to the resistor R. 
A voltage comparator circuit includes resistors R.sub.2 and R.sub.3, and a 
known differential operational amplifier OP.sub.1 incorporating integrated 
circuit. The resistors R.sub.2 and R.sub.3 serve to set the aforementioned 
reference voltage. Two input terminals of the operational amplifier 
OP.sub.1 are connected, respectively, to the junction between the detector 
resistor R and the reference resistor R.sub.1 and to the junction between 
the resistors R.sub.2 and R.sub.3. The operational amplifier OP.sub.1 
compares the voltages at the above two junctions and issues at the output 
terminal an electric signal of logic "1" or "0" level. More particularly, 
logic "1" signal is produced when the detected air-fuel ratio is smaller 
than the stoichiometrical air-fuel ratio, and logic "0" signal is produced 
when the former is larger than the latter. 
In this connection, it should be noted that the catalyst 15 carried in the 
insulation coating or layer 14 covering the surface of the gas component 
detecting element 13 exposed to the exhaust gas will serve to generate 
reaction heat upon the reaction of O.sub.2 with CO or HC, as a result of 
which surface temperature of the detecting element 13 is increased. As a 
consequence, even when the temperature of exhaust gas is low, as at the 
time of starting the internal combustion engine, the sensitivity of the 
gas component detecting element 13 will not be reduced. 
In the first embodiment described, since the heat-resisting porous 
insulation coating or layer 14 having exhaust gas permeability covers the 
surface of the gas component detecting element 13 exposed to the exhaust 
gas, the poisonous substances such as P, Pb and S contained in the exhaust 
gas will scarcely adhere directly to that surface of the element 13. 
Accordingly, there will scarcely happen the case where the poisonous 
substances react with the semiconductive metal oxide constituting the 
detecting element 13 and thereby forming compounds. Thus, the 
characteristic of the detecting element 13 as a semiconductor can be 
maintained stable for a long period. Further, since the insulation layer 
14 is porous and thus has the exhaust gas permeability, the exhaust gas 
can adequately contact the detecting element 13. In this connection, it 
should be noted that the catalyst 15 is carried and diffused in the porous 
insulation layer 14, so that the total active area of the catalyst 15 is 
fairy increased. Consequently, the oxidation reactions among the gas 
components are accelerated, and thus the sensitivity of the gas component 
detecting element 13 to the variation in the concentrations of the gas 
components is extremely improved. The insulation layer 14 is effective 
also to protect the catalyst 15 from being deteriorated due to the action 
of the poisonous substances described above. 
FIG. 7 shows a second embodiment of the present invention, in which the 
detecting element 13 is constituted by a disk-like body composed of a 
sintered semiconductive metal oxide such as TiO.sub.2. The disk-like 
detecting element 13 has a diameter of about 8 mm and a thickness of about 
4 mm. The first and the second electrodes 10 and 11 are disposed within 
the detecting element 13 adjacent the surface thereof over which the 
insulation layer 14 is applied. The insulation layer 14 carries the 
catalyst 15 consisting of the catalyst substances such as Pt. 
FIGS. 8 and 9 show a third embodiment of the present invention. As is the 
case of the second embodiment, the gas component detecting element 13 of 
the third embodiment is composed of a sintered semiconductive metal oxide 
such as TiO.sub.2. The surface of the detecting element 13 exposed to the 
exhaust gas is coated and covered with the porous insulation layer 14 made 
of heat-resisting materials such as .gamma.-Al.sub.2 O.sub.3 and has the 
exhaust gas permeability. The first and the second electrodes 10 and 11 
are disposed on the surface of the detecting element 13 over which the 
insulation layer 14 is coated. As do the first and the second embodiments, 
the insulation layer 14 carries the catalyst 15 consisting of Pt or the 
like. Over the surface of the insulation layer 14 exposed to the exhaust 
gas is coated with an additional insulation layer 23 composed of the same 
or similar materials as those of the insulation layer 14. In FIG. 9, 23a 
designates the particles constituting the insulation layer 23. 
FIG. 10 shows a main portion of a fourth embodiment of the present 
invention. The fourth embodiment is different from the third embodiment 
merely in that the first and the second electrodes 10 and 11 are disposed 
within the detecting element 13 at locations several microns spaced 
inwards (upward in FIG. 10) from the surface of the detecting element 13 
over which the insulation layer 14 is coated. 
FIGS. 11 and 12 show a main portion of a fifth embodiment of the invention. 
Also in this embodiment, the gas component detecting element 13 is 
composed of a sintered semiconductive metal oxide such as TiO.sub.2. The 
electrodes 10 and 11 are disposed within the detecting element 13 at 
locations several microns inwards from the surface of the detecting 
element 13 exposed to the exhaust gas. Over the exposed surface of the 
detecting element 13 is carried a catalyst 24 consisting of catalyst 
substances such as Pt. The porous insulation layer 14 composed of 
heat-resisting material such as .gamma.-Al.sub.2 O.sub.3 and having the 
exhaust gas permeability covers the exposed surface of the catalyst 24. As 
does the previous embodiments, the insulation layer 14 carries the 
catalyst 15. In FIG. 12, reference character 24a denotes the particles 
constituting the catalyst 24. 
A sixth embodiment shown in FIGS. 13 and 14 is differed from the fifth 
embodiment shown in FIGS. 11 and 12 in that the surface of the insulation 
layer 14 exposed to the exhaust gas is coated with an additional 
insulation layer 23 of the same material as that of the insulation layer 
14. 
FIG. 15 shows a seventh embodiment of the invention. In this embodiment, 
the gas component detecting element 13 consists similarly of a sintered 
mass of transition metal oxides such as TiO.sub.2. The first and the 
second electrodes 10 and 11 are disposed on the surface of the detecting 
element 13 exposed to the exhaust gas. The catalyst 24 is carried on the 
latter surface of the detecting element 13 in a manner to prevent 
formation of a short-circuit between the electrodes 10 and 11. The exposed 
surface of the catalyst 24 in turn is deposited with a porous insulation 
layer 14 composed of heat-resisting materials such as .gamma.-Al.sub.2 
O.sub.3 and having the exhaust gas permeability. The insulation layer 14 
carries the catalyst 15. 
FIG. 16 shows a main portion of an eighth embodiment of the invention which 
is different from the seventh embodiment of FIG. 15 in that the exposed 
surface of the insulation layer 14 carrying the catalyst 15 is coated with 
another insulation layer 23 of the same material as that of the former. 
FIGS. 17 and 18 show a ninth embodiment of the invention which is 
substantially similar in construction to the fifth embodiment shown in 
FIGS. 11 and 12, except that the catalyst 24 is diffused and carried in 
the gas component detecting element 13 in the vicinity of the exposed 
surface thereof. The catalyst 24 is effective to prevent the electrodes 10 
and 11 from being short-circuited. 
In the tenth embodiment shown in FIGS. 19 and 20, the exposed surface of 
the insulation layer 14 is coated with another insulation layer 23 of the 
same material as that of the former. 
FIG. 21 shows an eleventh embodiment of the invention which is 
substantially identical in construction with the seventh embodiment, 
except that the catalyst 24 is diffused and carried in the gas component 
detecting element 13 adjacent the exposed surface thereof. Also in this 
embodiment, the catalyst 24 serves to prevent formation of a short-circuit 
between the electodes 10 and 11. 
FIG. 22 shows a twelfth embodiment which is differed from the eleventh 
embodiment in that the surface of the insulation layer 14 exposed to the 
exhaust gas is coated with another insulation layer 23 of the same 
material. 
FIG. 23 shows a thirteenth embodiment of the invention. In this embodiment, 
the gas component detecting element 13 is made of a semiconductive metal 
oxide such as TiO.sub.2 in a form of a thin film. The catalyst 15 is 
directly carried on the surface of the detecting element 13 exposed to the 
exhaust gas. The exposed surface of the catalyst 15 is coated with the 
insulation layer 14 formed of .gamma.-Al.sub.2 O.sub.3. 
According to a fourteenth embodiment of the invention shown in FIG. 24, the 
gas component detecting element 13 formed of a sintered mass of 
semiconductor metal oxides such as TiO.sub.2 is directly deposited on its 
exposed surface with the catalyst 15 so as to prevent formation of a 
short-circuit between the electrodes 10 and 11. The catalyst 15 is coated 
with the insulation layer 14 formed of .gamma.-Al.sub.2 O.sub.3. 
In a fifteenth embodiment of the invention shown in FIG. 25, a sintered 
mass of TiO.sub.2 which is a transition metal oxide is used as the gas 
component detecting element 13. On the surface of the element 13 exposed 
to the exhaust gas is disposed the first electrode 10 and the catalyst 15. 
The electrode 10 and the catalyst 15 are coated with the insulation layer 
14. The variation in the electric resistance of the detecting element 13 
is take out through the lead wire 8 electrically connected to the first 
electrode 10 and the housing 1 electrically connected through the metal 
ring 4 to the second electrode 11 which is attached to the tapered edge 
portion of the detecting element 13. 
Referring to FIG. 26, there is graphically illustrated a relationship 
between the response time of change in the electric resistance of the 
detecting element 13 in response to the variation in the gas component 
concentration and the positions of the first and the second electrodes 10 
and 11. It can be seen from this figure that the response time of the gas 
component detecting element 13 or the time required for the element 13 
changing its resistance value in response to the variation in 
concentrations of the gas components can be remarkably decreased by 
positioning the first and the second electrodes 10 and 11 on or adjacent 
the surface of the detecting element 13 which is to be exposed to the 
exhaust gas. 
In the thirteenth embodiment described hereinbefore, the gas component 
detecting element 13 has been made of TiO.sub.2, ZnO, SnO .sub.2 and 
Nb.sub.2 O.sub.3 (niobium oxide) which are N-type semiconductor materials 
and NiO a P-type semiconductor material. Then, the characteristics of 
electric resistances of these detecting elements relative to the air-fuel 
ratio and the temperature have been measured. The results are graphically 
illustrated in FIGS. 27 to 31, respectively. It can been seen from these 
graphs that the electric resistance of the detecting element will change 
rapidly in the vicinity of the stoichiometrical air-fuel ratio and that 
the electric resistance is not subjected to the influence of the 
temperature of the exhaust gas in the region near the stoichiometrical 
air-fuel ratio. In these graphs, the electric resistance (K .OMEGA.) of 
the gas component detecting element 13 is taken along the ordinate in a 
logarithm scale, while the air-fuel ratio (A/F) is taken along the 
abscissa in equalized division. 
It will be understood from the above fact that the compensation for the 
exhaust gas temperature will be unnecessary if the detection threshold 
level is set with high accuracy. Thus, the gas component detection 
apparatus according to the invention can be advantageously utilized for 
the control of the air-fuel ratio in the internal combustion engine of 
motor cars in which the temperature of the exhaust gas undergoes large 
variation in dependence upon the operating or running conditions. 
It is also noticed that the detection of the air-fuel ratio can be effected 
without appreciable errors since the electric resistance varies abruptly 
in the neighborhood of the stoichiometrical air-fuel ratio. 
It is further to be noted that, in the case of the fifth, seventh, ninth 
and eleventh embodiments of the gas component detecting apparatus 
according to the invention in which the catalyst 24 is deposited on the 
surface of the gas component detecting element 13 or diffused therein 
adjacent that surface in addition to the catalyst 15 carried by the 
insulation layer 14, the sensitivity of the detecting element 13 to the 
variation in concentrations of gas components can be protected from being 
lowered by virtue of the presence of catalyst 24, even if the catalyst 15 
carried by the insulation layer 14 is degraded due to the action of 
poisonous substances contained in the exhaust gas. 
On the other hand, in the case of the third, fourth, sixth, eighth, tenth 
and twelfth embodiments of the invention in which the insulation layer 14 
carrying the catalyst 15 is coated with the additional insulation layer 
23, a large part of poisonous substances contained in the exhaust gas is 
caught by the outermost insulation layer 23 and the remaining part of the 
poisonous substances escaped from the insulation layer 23 is then caught 
by the inner insulation layer 14. Thus, the poisonous substances are 
effectively prevented from being deposited or accumulated on the surface 
of the detecting element 13. This feature also contributes to the 
prevention of deterioration of the catalyst 15. 
With a view to clarify the advantageousnesses of the detection apparatus 
according to the invention over the hitherto known detecting apparatuses, 
durability tests have been conducted to show how the characteristic of 
resistance variation in response to the variation in the air-fuel ratio as 
well as the response time of the detecting element to the variation in the 
concentration of gas components are changed after use of the detection 
apparatus for a long period. On the test, the detection apparatus 
according to the first, third, seventh and the eighth embodiments of the 
invention have been used along with two conventional detecting apparatuses 
having no insulation layer, one of which has catalyst carried directly on 
the surface of the detecting element exposed to the exhaust gas, while the 
other has no catalyst. 
The measuring conditions and the durability test conditions are summarized 
as follows; 
______________________________________ 
(1) Measuring conditions: 
Four-cylinder engine of 2000 (c.c.) capacity 
(having fuel injection apparatus) 
Engine speed: 2000 r.p.m. 
Negative pressure in the intake manifold: -240 mmHg 
Temperature of exhaust gas: 590.degree. C 
Gasoline used: lead-free gasoline (lead 
content: 0.02 g/gallon) 
(2) Durability test conditions: 
Duration: 100 hours 
Four-cylinder, 1600 (c.c.) engine (with a 
carburetor) 
Engine speed: 3000 r.p.m. 
Negative pressure in the intake manifold: -240 mmHg 
Gasoline used: lead-free gasoline (lead 
content: 0.02 g/gallon) 
______________________________________ 
The materials or substances of the gas component detecting portions 
according to the first, third, seventh and the eighth embodiments of the 
invention as well as the prior detectors are listed up in the following 
Table 1. 
Table 1 
__________________________________________________________________________ 
MATERIALS and DIMENSIONS 
Lead Wires 
Electrodes 
Detecting 
Insulation 
Catalyst 
Insulation 
Catalyst 
Examples 
(8, 9) 
(10, 11) 
Element (13) 
Layer (14) 
(15) Layer (23) 
(24) 
__________________________________________________________________________ 
Embodiment 1 
Pt (0.5 .phi.) 
Pt TiO.sub.2 
.gamma.-Al.sub.2 O.sub.3 
Pt None None 
(2 mm thick) 
.gamma.-Al.sub.2 O.sub.2 
Embodiment 3 
Same Same Same Same Same (0.2 mm 
None 
thick) 
Embodiment 7 
Same Same Same Same Same None Pt 
.gamma.-Al.sub.2 O.sub.3 
Embodiment 8 
Same Same Same Same Same (0.2 mm 
Same 
thick) 
Prior Art 1 
Same Same Same None Same None None 
Prior Art 2 
Same Same Same None None None None 
__________________________________________________________________________ 
The insulation layers 14 and 23 have been formed by applying slurries of 
.gamma.-Al.sub.2 O.sub.3 and thereafter firing the same; and the catalysts 
15 and 24 have been deposited on the gas component detecting element 13 by 
immersing the element 13 in the solution of H.sub.2 PtCl.sub.4.6H.sub.2 O, 
reducing with hydrogen and finally firing it. 
The change in the response time after the durability test is listed up in 
Table 2 and the change in the variation characteristic of electric 
resistances of the detecting apparatuses according to the invention after 
the durability test is shown in FIGS. 32 and 33. 
Table 2 
______________________________________ 
Response Times 
Initial After use 
Examples (new part) (100 hrs.) 
______________________________________ 
Embodiment 1 200 260 
(m. sec.) (m. sec.) 
Embodiment 3 230 250 
Embodiment 7 160 230 
Embodiment 8 180 200 
Prior Art 1 140 540 
Prior Art 2 600 1000 
______________________________________ 
As will be clearly understood from the above Table 2 and the graphs shown 
in FIGS. 32 and 33, the gas component detection apparatus according to the 
invention exhibits excellent performances as compared with the hitherto 
known detection apparatuses. For example, the response times of the 
conventional detection apparatus after the durability test is remarkably 
increased, as can be seen from Table 2. This means that the response of 
the detection apparatus can not immediately follow the variation in the 
air-fuel ratio in the control thereof. Further, there is an undesirable 
possibility in the hitherto known detectors that, even when the detected 
air-fuel ratio has changed from a high value to a low value, the detection 
apparatus may issue an error signal representing a high air-fuel ratio due 
to the delayed response of the apparatus. In contrast, in the case of the 
gas component detection apparatus according to the invention, the response 
time after the durability test has not been significantly changed from the 
initial value and remains short as compared with that of the conventional 
detection apparatus. Further, the variation characteristics of electric 
resistance of the detecting element remain substantially unchanged after 
the durability test, as can be seen from FIGS. 32 and 33. The gas 
component detection apparatus according to the invention is thus immune to 
the drawbacks of the conventional detection apparatus described above. 
Although the results of the durability test for the second, fourth, fifth, 
sixth, ninth, tenth, eleventh, twelfth, thirteenth, fourteenth and 
fifteenth embodiments have not been shown herein, it has been found that 
the detection apparatus according to these embodiments are also far 
excellent over the hitherto known detection apparatus as exemplified by 
the prior arts 1 and 2 in the above tables in respect of the performance. 
Although the material of the outermost insulation layer 23 of the third, 
fourth, eighth, tenth and the twelfth embodiments is same as the 
insulation layer 14, it is of course possible to use a different material. 
In essence, it is sufficient that the insulation layers or films 14 and 23 
are composed of a porous metal oxide having a heat resistivity and an 
exhaust gas permeability. 
Although the invention has been described with reference to the gas 
component detection apparatus applied to detecting the air-fuel ratio of 
the air-fuel mixture supplied to the internal combustion engine, it should 
be appreciated that the invention is never restricted to such application. 
For example, the invention can be applied to detecting the concentration 
of gas components (combustion products) exhausted from a combustion 
apparatus of a blast furnace in order to control the air-fuel ratio of the 
combustible mixture supplied to the combustion apparatus. 
As will be appreciated from the foregoing description, in the gas component 
detection apparatus according the present invention, the electric 
resistance of the detecting element varies rapidly and significantly under 
action of the catalyst when the detected air-fuel ratio is deviated from a 
certain reference value, i.e. the stoichiometrical air-fuel ratio. 
Accordingly, it is possible to detect the airfuel ratio with a minimum 
error and a high accuracy. Further, the detection can be effected without 
being influenced by temperature of the detected gas. Besides, the 
temperature of the gas component detecting element can be raised rapidly 
due to the heat generated by the reaction with the gas component under the 
action of catalyst, which contributes to an enhancement of response of the 
detecting element. 
When poisonous substances such as P, S, Pb and C are contained in the 
detected gas, these substances are trapped by the insulation layer(s) 
without adhering directly to the surface of the gas component detecting 
element, whereby the possibility of the transition metal oxide of the 
detecting element reacting with the poisonous substances to form compounds 
is effectively suppressed. These compounds may possibly involve the 
deterioration of the semiconductor property of the transition metal oxide. 
Thus, the performance or the semiconductor property of the gas component 
detecting element can be maintained in a stabilized condition for a long 
use life. The porous insulation layer aids to increase the active surface 
area of the catalyst carried therein thereby to enhance the sensitivity of 
the gas component detecting element to the variation in the concentrations 
of gas components contained in the detected gas. The catalyst is also 
protected from being deteriorated by virtue of the presence of the 
insulation layer. 
According to the invention, the electrodes for sensing the resistance 
variation of the detecting element is located on the surface of the 
element coated with the insulation layer or disposed within the element 
adjacent that surface. Such arrangement assures a rapid response of the 
detecting element to the concentration variation of the gas components, 
and allows a detection of the concentration of gas components with high 
accuracy. 
Although the invention has been described with reference to preferred 
embodiments shown in the drawings, it will be appreciated that the 
invention is never restricted to them. Those skilled in the art can easily 
conceive many modifications without departing from the spirit and the 
scope of the invention.