Oxygen sensor

An oxygen sensor for monitoring the oxygen concentration in exhaust gases of an engine of a motor vehicle including a sensor element supported by a base in a position remote from the flow of exhaust gases to be tested whereby solid particles in the exhaust gases are prevented from being deposited on the sensor element and the sensor element is imprevious to influences exerted by the temperature and flow velocity of the flow of exhaust gases. The oxygen sensor is capable of stably monitoring the oxygen concentration in exhaust gases of an engine of a motor vehicle with a high degree of performance over a prolonged period of time.

BACKGROUND OF THE INVENTION 
Field of the Invention 
This invention relates to oxygen sensors, and, more particularly, to an 
oxygen sensor suitable for use in monitoring the concentration of oxygen 
in gases containing minute solid particles, such as exhaust emissions of 
motor vehicles. 
One type of an oxygen sensor known in the art is based on the principles of 
oxygen concentration cell or oxygen pump in solid electrolytes, such as 
zirconia. Another type known in the art uses the phenomenon of 
transmission of oxygen ions in metal oxides, such as titania. It is well 
known that these oxygen sensors of the prior art are 
temperature-dependent. Proposals have been made to provide improvements to 
these temperature-dependent oxygen sensors, with one such proposal 
disclosed, for example, in Japanese Laid Open Application No. 166252/83 
including the provision of a heating element located close to the sensor 
element to minimize influences which might be exerted on the sensor by the 
temperature of gases to be tested. 
In order to achieve improved fuel efficiency and engine performance while 
avoiding environmental pollution by exhaust emissions by effecting finer 
control of fuel-air mixtures supplied to the engine of an automotive 
vehicle, there has been a demand to widen the range of operation of the 
oxygen sensors of the type referred to hereinbefore from the so-called 
theoretical air-fuel ratio to the entire range of working air-fuel ratios, 
particularly to lean mixtures. To meet this demand, it is necessary that 
the sensor element be controlled at a higher temperature (at a constant 
value in the range between 300.degree. C. and about 800.degree. C. at 
which the point of theoretical air-fuel ratio is sensed), in view of the 
physical properties of its material. The exhaust pipe of an automotive 
vehicle in which the oxygen sensor is mounted shows great fluctuations in 
temperature between -50.degree. and +800.degree. C. and in the velocity of 
air currents between 0 and 100 m/sec. This makes it necessary to pass an 
electric current to the oxygen sensor to heat same nearly at all times. 
However, to achieve economy in fuel consumption, it would be necessary to 
minimize electric power used for the purpose of heating. This would make 
it necessary to minimize the thermal capacity of the sensor and adopt 
support means which could inhibit transfer of heat to a portion of the 
exhaust pipe in which the sensor is mounted. As a result, the sensor 
including a support member has generally become elongated in shape, and 
proposals have been made to use a sensor element of small volume which may 
be either plate-like, disc-like or film-like in form, as disclosed, for 
example, in Japanese Patent Application Laid-Open No. 42965/83. 
On the other hand, as noted in Automotive Vehicle Technology, 1972, Vol. 
26, No. 9, exhaust emissions contain solid particles of carbon, magnesium, 
silicon, phosphorus, sulfur, calcium, chromium, iron, zinc, lead, etc., 
existing in the form of grit produced by the combustion reaction between 
sucked air and fuel or lubricant and the sliding contact between engine 
cylinders and pistons and between suction or discharge valves and the 
valve seats, and these solid particles are known to be deposited on walls 
and other parts of the exhaust pipes. In these deposits, lead, zinc, iron, 
chromium and calcium, relatively heavy in weight, remain in a solid state 
without vaporizing even if they are heated to 800.degree. C. Thus, these 
elements remain deposited on the surface of the sensor element even if the 
latter is heated to a temperature of 800.degree. C., making it impossible 
for the sensor element to perform monitoring due to the fact that the 
bores on the surface of the element for diffusing gases are blocked by the 
deposits or the triphasic interface of an electrode is covered with them. 
It is known that the smaller the size of the sensor element, the greater 
the influences exerted by the solid particles on the results achieved by 
the sensor. 
This invention has as its object the provision of an oxygen sensor capable 
of maintaining its capacity to monitor the concentration of oxygen at a 
desired level over a very long time even if it is installed in a gas flow 
which contains solid particles and shows great fluctuations in temperature 
and flow velocity. 
To accomplish the aforesaid object, the invention provides an oxygen sensor 
capable of reducing or minimizing influences which might be directly 
exerted by the gas flow on the sensor element for monitoring the 
concentration of oxygen in the gas. 
According to the invention, the sensor element is enclosed by double 
cylinders including an outer cylindrical member and an inner cylindrical 
member, wherein a gas flow, admitted to the interior of the outer 
cylindrical member through openings formed in the outer cylindrical 
member, impinges on an outer wall surface of the inner cylindrical member 
so that solid particles in the gas are deposited thereon, and the gas flow 
is admitted to the interior of the inner cylindrical member through 
openings formed in the inner cylindrical member in positions in which the 
gas flow is out of alignment with the sensor element, to thereby reduce 
the flow velocity of the gas flow in a vicinity of the sensor element and 
render the latter imprevious to the influences of the gas flow. 
According to further features of the invention, the sensor element is 
housed in a protective metal member of substantially cylindrical 
configuration which is closed at the bottom and formed with openings to 
allow a gas flow to enter into and exit from the protective metal member 
in positions remote from the sensor element in the protective metal 
member, whereby the majority of the gas flow entering the protective metal 
member passes through the vicinity of the supporting portion of the sensor 
which is near the inner wall surface of a passage of the tested gases and 
direct influences which might be exerted on the sensor element by the gas 
flow can be reduced.

DETAILED DESCRIPTION 
Preferred embodiments of the oxygen sensor in conformity with the invention 
will now be described by referring to the accompanying drawings. 
Referring now to the drawings wherein like reference numerals are used 
throughout the various views to designate like parts and, more 
particularly, to FIG. 1, according to this figure the oxygen sensor 
comprises a base 1 of a rectangular plate shape having an increased width 
portion 11 serving as a partially-stabilized zirconium solid electrolyte. 
The base 1 supports, at its lower end portion, a sensor element 2 having a 
built-in electrical-heating element serving concurrently as a 
temperature-responsive element, and, at its upper end, conductive 
terminals 6 for the sensor element 2 and electrically-heating element 
which are connected to a control circuit, not shown. The base 1 is 
supported by a support metal member 3 of substantially cylindrical 
configuration formed with slots and closed at its bottom in such a manner 
that the conductive terminals 6, located at the increased width portion 
11, stick out the support metal member 3, with an airtight sealing member 
9, such as aluminum oxide powder, talc powder or an inorganic adhesive 
agent filling a gap between the base 1 and the support metal member 3 to 
firmly secure the former to the latter. 
The oxygen sensor comprises an outer metal member 8 which is of 
substantially cylindrical configuration and closed at its bottom and which 
is formed at its peripheral wall with eight louvered slots 8a and at its 
bottom with four circular openings 8d. The oxygen sensor comprises an 
inner metal member 7, substantially cylindrical in configuration, closed 
at its bottom and spread outwardly at its open top end portion which is 
formed with eight claws 7a and eight cutouts 7b arranged in the form of a 
rosette, which is inserted in the outer metal member 8 and joined at its 
bottom to a bottom wall 8c of the outer metal member 8 by spot welding. 
When the inner metal member 7 is fitted in the outer metal member 8, the 
cutouts 7b and the louvered slots 8a are out of alignment with each other 
as viewed in a longitudinal direction of the sensor as shown in FIG. 1, 
and the claws 7a are maintained at their outer edges in contact with an 
inner surface of the peripheral wall of the outer metal member 8 while the 
circular openings 8d are not closed by the bottom of the inner metal 
member 7. The outer metal member 8 is spread outwardly at its open top end 
portion to provide a flange 8e which is fitted to a stepped circular edge 
portion 4a of a mounting metal member 4 after the cylindrical outer metal 
member 8 is inserted in a circular opening 4b of the mounting metal member 
4. A flange 3a of the support metal member 3 is fitted in the flange 8e, 
and a disc-shaped lid 5 formed with slots is fitted in the flange 3a and 
joined by welding at its entire circumference to an inner surface of the 
flange 3a to provide the oxygen sensor which, in its completed form, is 
mounted to the mounting metal member 4. After the oxygen sensor is thus 
completed, the sensor element 2 of the oxygen sensor is located in such a 
manner that it is out of alignment with the cutouts 7b formed in the inner 
metal member 7 lengthwise of the oxygen sensor as shown in FIG. 1. 
The oxygen sensor of the invention described hereinabove is fitted to an 
exhaust pipe, not shown, of an engine of a motor vehicle by the mounting 
metal member 4. As the conductive terminals 6 are connected to the power 
source of the control circuit and the louvered slots 8a are exposed to a 
flow of exhaust gas in and through the exhaust pipe, the exhaust gases 
find their way into the outer metal member 8 through the louvered slots 
8a, and main currents of gas flow in vertical movement between the inner 
and outer metal members 7, 8 before being released to outside through the 
louvered slots 8a. A portion of the gas flowing into the outer metal 
member 8 flows in two directions longitudinally of the oxygen sensor, 
namely, the gas flowing in one direction flows out of the circular 
openings 8d at the bottom of the outer metal member 8, and the gas flowing 
in the other direction flows through the cutouts 7b into the inner metal 
member 7 and the majority of gases flows out of the inner metal member 7 
into the outer metal member 8. 
Accordingly, the solid particles contained in the currents of exhaust 
gases, particularly those which are heavy in weight, are blocked in their 
flow by the outer wall surface of the outer metal member 8, and trapped 
between the outer wall surface of the inner metal member 7 and the inner 
wall surface of the outer metal member 8 while only those solid particles 
which are light in weight find their way into the inner metal member 7 
through the cutouts 7b and nearly all of them flow out of the inner metal 
member 7 into the outer metal member 8 through the cutouts 7b. Thus, 
almost no solid particles are brought into contact with the sensor element 
2 remote from the cutouts 7b, with only a portion of the particles, very 
light in weight, such as carbon which is flammable, swirling in the inner 
metal member 7. Since the sensor element 2 is maintained at a high 
temperature of 800.degree. C. by the builtin electrical-heating element, 
the swirling solid particles are combusted and not allowed to be deposited 
on the sensor. The circular openings 8d at the bottom of the outer metal 
member 8 serve the purpose of releasing to outside from the outer metal 
member 8 the solid particles which impinge on the outer wall surface of 
the inner metal member 7 and the inner wall surface of the outer metal 
member 8. 
In the embodiment of the oxygen sensor of the construction described 
hereinabove, it is possible to advantageously avoid fluctuations in 
temperature to which the sensor element 2 might be exposed. More 
specifically, the outer metal member 8 has the effect of reducing changes 
in the temperature of exhaust gases flowing through the exhaust pipe, and 
the gases flowing through the louvered slots 8a into the outer metal 
member 8 have changes in temperature reduced by the inner metal member 7. 
The difficulty with which the energy of the exhaust gases imparted to the 
inner metal member 7 is transferred to the support metal member 3 of low 
temperature further reduces changes in the temperature of the exhaust 
gases. Thus, the changes in temperature to which the sensor element 2 
disposed in the lower portion of the inner metal member 7 is exposed can 
be greatly reduced. 
The embodiment of the oxygen sensor of the aforesaid construction is also 
capable of avoiding fluctuations in the flow velocity of exhaust gases to 
which the sensor element 2 is exposed. More specifically, as the flow 
velocity increases, the flow of exhaust gases admitted to the outer metal 
member 8 through the louvered slots 8a swirls by the louvers and is 
released to outside through the louvered slots 8a after moving 
peripherally in the outer metal member 8 at increased flow velocity, and 
the flow of exhaust gases in the outer metal member 8, oriented 
longitudinally in two directions, is not appreciably accelerated. Thus, 
the flow velocity of gases admitted to the inner metal member 7 through 
the cutouts 7b, much less that of gases flowing in the vicinity of the 
sensor element 2, shows no great changes. It will be appreciated that, 
although the flow velocity of exhaust gases flowing outside the outer 
metal member 8 shows great fluctuations, the flow velocity of exhaust 
gases in the vicinity of the sensor element 2 is not greatly affected by 
the fluctuations in the flow velocity of exhaust gases outside the outer 
metal member 8 and remains substantially constant at all times. 
In the embodiment shown in FIGS. 1-4, the open end portion of the 
cylindrical inner metal member 7 closed at the bottom is spread outwardly 
and the cutouts 7b, serving as gas ports, are formed in the outwardly 
spread open end portion of the member 7. However, this is not restrictive, 
and cutouts may be formed without outwardly spreading the open end portion 
of the member 7. Also, the inner metal member 7 need not be cylindrical in 
form so long as it is closed at the bottom and open at the top, and the 
invention is not limited to the specific shape and number of the cutouts 
7b. What is important is that the gas ports be formed in positions in 
which they do not face the sensor element 2 and the flow velocity of gases 
be made relatively lower in positions near the sensor element 2 than in 
positions remote from the sensor element 2. 
In the embodiment of the oxygen sensor shown in FIGS. 1-4, the outer metal 
member 8 of substantially cylindrical configuration closed at its bottom 
is formed with eight louvered slots 8a at its peripheral wall and four 
circular openings 8d at its bottom. However, the outer metal member 8 may 
be cylindrical in configuration without a closed bottom, and may be formed 
at its peripheral wall with openings other than the louvered slots. The 
outer metal member 8 may be formed at its bottom with openings which are 
not circular in shape, and the openings at the bottom may be dispensed 
with or the bottom may be open without being closed. What is important is 
that the gas ports formed in the outer metal member 8 be not essentially 
aligned with the gas ports formed in the inner metal member. This 
arrangement of the gas ports in the outer and inner metal members is 
covered by the scope of the invention. 
In FIGS. 5 and 6, the oxygen sensor comprises a base 1a of substantially 
rectangular plate-like configuration formed of the same electrolyte as the 
base 1 of the first embodiment, and the sensor element 2 is mounted at one 
end portion of the base 1a which supports at an opposite end portion wires 
for electrically connecting the sensor element 2 and heating element with 
a harness 16. The base 1a is supported at its intermediate portion by a 
support metal member 13 having a flange, and air-tightly sealed as is the 
case with the base 1 of the first embodiment. 
A protective metal member 12 of substantially tubular configuration is 
closed at its bottom and formed with a flange at an open end portion, with 
the protective metal member 12 being fitted in a stepped opening of a 
metal flange 14 and joined to the support metal member 13 by welding. An 
outer metal member 15 of substantially cylindrical configuration is formed 
with a flange at an open end of its major diameter portion which is fitted 
in the stepped opening of the metal flange 14 and joined thereto by 
welding. The outer metal member 15 is clamped at its minor diameter 
portion to support the harness 16. The protective metal member 12 is 
formed with gas ports 12a, 12b, 12c and 12d at its peripheral wall near 
the flange, and with a gas port 12e of a smaller diameter than the gas 
ports 12a-12d at its bottom wall near the sensor element 2. The parts 
described hereinabove constitute a main body of the oxygen sensor. A 
sensor mount 18 having stud bolts 19, is attached to an exhaust pipe 17 of 
the motor vehicle, and the main body of the oxygen sensor is mounted on 
the sensor mount 18 through a packing 21, and a nut 20 is threadably 
fitted to each stud bolt 19. When the main body of the oxygen sensor is 
fitted to the exhaust pipe 17 as described hereinabove, the gas ports 
12a-12d of the protective member 12, formed at its peripheral wall, are 
located in the immediate vicinity of an inner wall surface of the exhaust 
pipe 17 and the sensor element 2 is located in a position remote from the 
gas ports 12a-12d at the peripheral wall of the protective metal member 
12. When the exhaust gases flow from left to right in FIG. 5, the flow of 
exhaust gases through the exhaust pipe 17 is shed by the peripheral wall 
of the protective metal member 12 and a portion of the exhaust gases is 
admitted through the gas port 12a at the peripheral wall thereof. The 
major portion of the exhaust gases, admitted through the gas port 12a, is 
released to outside through the gas port 12b diametrically opposed to the 
gas port 12a while a small portion thereof is released to outside through 
the gas port 12e at the bottom of the protective metal member 12. 
As described in the background of the invention, the temperature and flow 
velocity of exhaust gases flowing through the exhaust pipe show great 
fluctuations between 50.degree. and 800.degree. C. and between 0 and 100 
m/sec., respectively, and the exhaust gases contain solid particles of 
metallic and non-metallic origin. In the embodiment shown in FIGS. 5 and 
6, the exhaust gases flow through the protective metal member 12 in a 
position remote from the sensor element 2, so that the majority of the 
solid particles, particularly those of metallic elements of heavy weight, 
is released through the gas port 12b by the flow of exhaust gases of high 
velocity without impinging on the surface of the sensor element 2 and 
adhering thereto. Only a small portion of the solid particles, 
particularly those of light weight, such as carbon particles, may possibly 
be released through the gas port 12e at the bottom of the protective metal 
member 12. However, since the sensor element 2 is maintained at a high 
temperature of 800.degree. C. by the heating element, the solid particles 
of light weight floating in the protective metal member 12 are prevented 
from being deposited on and adhering to the surface of the sensor element 
2. 
In the embodiment shown and described hereinabove, control of the 
temperature of the sensor element by means of the heating element is 
advantageously effected. More specifically, even if the temperature and 
flow velocity of exhaust gases flowing through the exhaust pipe 17 show 
great fluctuations, changes in the temperature and flow velocity of the 
exhaust gases in the protective metal member 12 are greatly reduced 
because the gas ports 12a-12d formed at the peripheral wall of the 
protective metal member 12 are located in the immediate vicinity of the 
inner wall surface of the exhaust pipe 17. Moreover, since the sensor 
element 2 is remote from the gas ports 12a-12d and the inner wall surface 
of the exhaust pipe 17, influences exerted on the sensor element 2 by the 
transfer of heat to and from the base 1a are greatly reduced. 
In the foregoing description, the flow of exhaust gases has been described 
as being oriented in one direction. However, in actual practice, the 
direction of flow of exhaust gases might undergo a change when the motor 
vehicle suddenly accelerates or decelerates. In this case, if no other gas 
ports were provided than the gas ports 12a-12d formed at the peripheral 
wall of the protective metal member 12, the flow velocity of exhaust gases 
in the protective metal member 12 would fluctuate greatly due to a pumping 
action, and the exhaust gases would show a fluctuation in flow velocity to 
a considerable degree in the lower portion of the protective metal member 
12 or in the vicinity of the sensor element 2, with a result that the 
output of the sensor element 2 might show a variation which has nothing to 
do with the concentration of oxygen and the solid particles might impinge 
on and adhere to the sensor element 2. However, the provision of the gas 
port 12e at the bottom wall of the protective metal member 12 in addition 
to the gas ports 12a-12d at the peripheral wall thereof has the effect of 
minimizing the pumping action occurring in the protective metal member 12, 
making it possible to prevent a change or a rise in the flow velocity of 
exhaust gases in the vicinity of the sensor element 2 and to keep solid 
particles from being deposited on the sensor element 2. The gas port 12e 
formed at the bottom wall is smaller in diameter than the gas ports 
12a-12d formed at the peripheral wall. Combined with static pressure, this 
reduces the volume of exhaust gases flowing through the gas port 12e to a 
level below the volume of exhaust gases flowing through the gas ports 
12a-12d in almost all the engine speed range of motor vehicles in which 
the flow velocity of exhaust gases is constant or shows a gradual 
reduction. What is essential is that the flow rate of exhaust gases 
through the gas port 12e formed at the bottom wall be substantially 
reduced to a level below the flow rate of exhaust gases through the gas 
ports 12a-12d formed at the peripheral wall. 
In the foregoing description, the base of the oxygen sensor has been 
described as having a rectangular plate shape and being provided with a 
heating element. It is to be understood that the invention can also have 
application in an oxygen sensor including a base of a shape different from 
the one described and having no heating element, such as a base in the 
form of a square post or a column, so long as the sensor element is 
located in a position remote from the wall of a passage of exhaust gases 
to be tested. The invention has the effect of preventing solid particles 
from being deposited on the sensor element supported by the base of the 
aforesaid shape while avoiding changes in temperature which the sensor 
element might undergo. 
When the oxygen sensor according to the invention is mounted in a position 
in which the flow of exhaust gases to be tested is constant in direction, 
the gas port 12e at the bottom wall or the gas ports 12c and 12d at the 
peripheral wall may be dispensed with. 
The gas ports 12a-12d at the peripheral wall and the gas port 12e at the 
bottom wall are circular in shape. However, the invention is not limited 
to this specific shape of the gas ports, and the gas ports 12a-12e may be 
of any shape as desired, such as square or rectangular. The gas ports have 
been described as being two to five in number. However, the invention is 
not limited to these specific numbers. Any arrangement whereby a flow of 
exhaust gases through the protective metal member 12 is made essentially 
remote from the sensor element 2 and near the inner wall surface of the 
exhaust pipe 17 is covered by the scope of the claim for a patent in the 
subject application. 
The second embodiment of the invention shown in FIGS. 5 and 6 enables a 
prevention of the deposition of solid particles in the exhaust gases on 
the sensor element to be avoided. Also, the influences exerted on the 
sensor element by fluctuations in the temperature and flow velocity of the 
exhaust gases monitored can be minimized. In addition, an error in the 
output of the sensor element which might be caused to occur by a change in 
the direction of flow of the exhaust gases monitored can be eliminated. 
Thus, the embodiment shown in FIGS. 5 and 6 is capable of performing the 
operation of monitoring the oxygen concentration in the exhaust gases 
stably over a prolonged period of time. 
In the embodiments shown in FIGS. 1-6, the oxygen sensor has been described 
as including a sensor element supported by a base of zirconium solid 
electrolyte of rectangular plate shape and having a built-in 
electrically-heating element. It is to be understood that the invention 
can achieve similar effects when applied to oxygen sensors in which no 
built-in electrically-heating element is provided and the sensor element 
and conductive base supporting the sensor element are separate entities 
and in which the sensor element and base are separate entities and their 
shapes are not plate-like, so long as the sensor element is located in a 
position remote from the position in which the sensor is supported. 
The present invention can achieve, in oxygen sensors including all the 
types of sensor elements for monitoring the concentration of oxygen, the 
following effects: first, solid particles in the exhaust gases tested can 
be prevented from being deposited on the sensor element; secondly, the 
sensor element is not influenced by fluctuations in the temperature of the 
exhaust gases tested; and thirdly, the sensor element is not influenced by 
changes in the flow velocity of the exhaust gases tested. These features 
enable the oxygen sensor according to the invention to perform, with a 
high degree of performance, the function of stably monitoring the 
concentration of oxygen in the exhaust gases of an engine of an automotive 
vehicle flowing through its exhaust pipe over a prolonged period of time.