OXYGEN SENSOR

Abstract: An oxygen sensor for monitoring the oxygen concentra­ tion in exhaust gases of an engine of an automotive vehicle including a sensor element (2) supported by a base (1) 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 (2) and the sensor element is imprevious to influences exerted by the temperature and flow velocity of the flow of exhaust gases. The oxygen sensor (2) is capable of stable monitoring the oxygen concentration in exhaust gases of an engine of an automotive vehicle with a high degree of performance over a prolonged period of time.

Description: This invention relates to oxygen sensors as referred to in the generic part of claim 1. particularly it is concerned with an oxygen sensor suitable for use in monitoring the concentration of oxygen in gases containing minute solid particles, such as exhaust emissions of automotive vehicles. DESCRIPTION OF THE PRIOR ART One type of oxygen sensor known in the art is of a type which is based on the principles or oxygen concentration cell or oxygen pump in solid electrolytes, such as zirconia. Another type known in the art is of a type which 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. One of them includes 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, as disclosed in JP-58166252, for example. On the other hand, 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 (see Automotive Vehicle Technology, 1972, Vol. 26, No. 9). In these deposits, lead, zinc, iron, chromium and calcium which are relatively heavy in weight remain in a solid state without vaporizing even if they are heated to 800°C. Thus, these elements remain deposited on the surface of the sensor element even if the latter is heated to a temperature of 800°C, making it impossible for the sensor element to perform monitoring owing 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. DE-A-23 48 505 discloses an electro-chemical sensor for determining the oxygen quantity in exhaust gases, especially in exhaust gases of internal combustion machines comprising an ion conducting solid electrolytic tube one end of which has a bottom whose inner surface has an electron conducting lead with a terminal portion and which is exposed to the surrounding air and whose outer surface has an electron conducting catalytic layer being grounded by means of a metallic sensor housing and facing the exhaust gases, the portion facing the exhaust gases being surrounded by protective means disposed with distance around the solid electrolytic tube and extending beyond the bottom extremity of the solid electrolytic tube and having at least one opening for the input and output of exhaust gases, respectively, the provided openings having diversion means which guide the exhaust gases towards the solid electrolytic tube but prevent a direct impingement of exhaust gases in that the protective means comprise two coaxial and with a certain distance disposed envelopes with openings, the openings of the outer envelope being staggered against the openings of the inner envelope in a way, that they do not overleap. SUMMARY OF THE INVENTION 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 gases flow which contains solid particles and shows great fluctuations in temperature and flow velocity. This object is accomplished with oxygen sensors according to the independent claim 1. Dependent claims are directed on preferred embodiments of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a vertical sectional view of the oxygen sensor comprising a first embodiment of the invention, showing its entire structure; Fig. 2 is a side view of the oxygen sensor shown in Fig. 1; Fig. 3 is a transverse sectional view of the oxygen sensor shown in Fig. 1, taken along the line III-III in Fig. 1; Fig. 4 is a transverse sectional view of the oxygen sensor shown in Fig. 1, taken along the line IV-IV in Fig. 2; DESCRIPTION OF THE PREFERRED EMBODIMENTS Preferred embodiments of the oxygen sensor in conformity with the invention will now be described by referring to the accompanying drawings. Referring to Fig. 1, 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 electrically-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 airtightly 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 lengthwise 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 8a 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 first embodiment of the oxygen sensor in conformity with the invention constructed as described hereinabove is fitted to an exhaust pipe, not shown, of an engine of an automotive 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 gases in and through the exhaust pipe, the exhuast gases find their way into the outer metal member 8 through the louvered slots 8a, and main currents of gases flow in vertical movement between the inner and outer metal members 7 and 8 before being released to outside through the louvered slots 8a. A portion of the gases flowing into the outer metal member 8 flows in two directions lengthwise of the oxygen sensor: the gases flowing in one direction flow out of the circular openings 8d at the bottom of the outer metal member 8, and the gases flowing in the other direction flow through the cutouts 7b into the inner metal member 7 and the majority of them 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 them which is 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°C by the built-in electrically-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 those 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 is made to swirl by the louvers and 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 which is 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 essential 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 essential 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 the embodiments shown in Figs. 1-4, 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 embodiment of the invention can achieve, in oxygen sensors including all the types of sensor elements for monitoring the concentration of oxygen, can achieve 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 monitoring the concentration of oxygen in the exhaust gases of an engine of an automotive vehicle flowing through its exhaust pipe stably over a prolonged period of time.

Claims: An oxygen sensor of a double cylinder type, comprising: a sensor element (2); an inner cylindrical member (7) formed with gas ports (7b), said inner cylindrical member (7) housing said sensor element (2); and an outer cylindrical member (8) enclosing said inner cylindrical member (7); wherein at least said inner cylindrical member (7) is closed at its bottom, characterized in that said gas ports (7b) are formed at a peripheral wall of said inner cylindrical member (7) in positions in which they are not in face-to-face relation to the sensor element (2). An oxygen sensor as claimed in claim 1, wherein gas ports (8a) are formed at a peripheral wall of said outer cylindrical member (8) in positions in which they are not in face-to-face relation to the gas ports (7b) formed at the peripheral wall of the inner cylindrical member (7). An oxygen sensor as claimed in claim 2, wherein said outer cylindrical member (8) is closed at its bottom. An oxygen sensor as claimed in claim 2 or 3, wherein said gas ports (8a) formed at the peripheral wall of the outer cylindrical member are in the form of louvered slots. An oxygen sensor as claimed in claim 2, wherein said outer cylindrical member (8) is closed at its bottom by a bottom wall (8c) formed with gas ports (8d). An oxygen sensor as claimed in claim 4, wherein said outer cylindrical member (8) is formed with gas ports (8d) at its bottom. An oxygen sensor as claimed in any one of claims 1 - 6, wherein said inner cylindrical member (7) is formed with gas ports at its bottom. An oxygen sensor as claimed in any one of claims 1 - 6, wherein an open end portion of said inner cylindrical member (7) is outwardly spread to provide an outwardly spread open end portion whereby the inner cylindrical member (7) is maintained at the outwardly spread open end portion in contact with an inner surface of the peripheral wall of the outer cylindrical member (8) and supported thereby. An oxygen sensor as claimed in any one of claims 1 - 6, wherein said inner cylindrical member (7) is firmly secured at its bottom to the bottom (8c) of said outer cylindrical member (8) and supported thereby. An oxygen sensor as claimed in any one of claims 1 - 6, wherein the gas ports (7c) formed at said inner cylindrical member (7) are located as said outwardly spread open end portion of the inner cylindrical member (7). An oxygen sensor as claimed in any one of claims 1 - 6, wherein a heat generating element is located in the immediate vicinity of the sensor element.

Inventors: TAKAHASHI, TORU, UENO, SADAYASU
Applicants: HITACHI, LTD.
Application year: 1985
Main classification: G01N27
IPC codes: G01N27