Oxygen sensor

Abstract: An oxygen sensor for monitoring the oxygen concentration 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 stably 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: BACKGROUND OF THE INVENTION FIELD OF THE INVENTION This invention relates to oxygen sensors, and 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 of 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 Japanese Patent Application Laid-Open No. 166252/83, for example. Meanwhile, in order to achieve improved fuel efficiency and engine performance while avoiding environmental disruption 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°C and about 800°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 and +800°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 in Japanese Patent Application Laid-Open No. 42965/83, 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. US-A-4 098 653 shows an oxygen sensor with a solid electrolyte tube extending in the protective member essentially over the whole length thereof. The openings of the protective member extend over the whole length of the protective member which is not covered by the base, directing the gas current straight to the surface of the sensor element. In such an arrangement, solid particles contained in the exhaust gas will directly hit the surface of the sensor element. GB-A-2 061 522 shows a measuring sensor for the constituents of gases in which the sensor element is coaxially surrounded by a heating element mounted on a carrier. This carrier has a single hole at its bottom through which gases are admitted to the sensor. Also here, gases entering through this hole may directly accede to the surface of the sensor element, so that soot and metal particles from a motor may adhere to it. 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 achieved by an oxygen sensor as defined in claim 1. Dependent claims relate to advantageous embodiments of the invention. According to an embodiment 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 gases 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 gases 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 gases flow can be reduced. Fig. 1 is a fragmentary vertical sectional view of the oxygen sensor according to an embodiment of the invention, shown in a cross section which is parallel to the base of the sensor; and Fig. 2 is a fragmentary vertical sectional view of the oxygen sensor similar to that shown in Fig. 1 but shown in a cross section perpendicular to the base of the sensor. DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the invention will now be described by referring to Figs. 1 and 2. In this embodiment, the oxygen sensor comprises a base 1a of substantially rectangular plate-like configuration formed of partially stabilized zirconia electrolyte, and the sensor element 2 is mounted at one end portion of the base la which supports at an opposite end portion wires for electrically connecting the sensor element 2 and heating element with a harness 16. The base la is supported at its intermediate portion by a support metal member 13 having a flange, and air-tightly sealed by a sealing member of alumina powder, talc powder or an inorganic adhesive agent filling a gap between the base 1 and the support metal member to firmly secure the former to the latter. The numeral 12 designates a protective metal member of substantially cup-shaped configuration and formed with a flange at an open end portion. The protective metal member 12 is fitted in a stepped opening of a metal flange 14 and joined to the support metal member 13 by welding. The numeral 15 designates an outer metal member of substantially cylindrical configuration 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. The numeral 17 designates a portion of an exhaust pipe of an engine of an automotive vehicle. A sensor mount 18 having stud bolts 19 is attached to the exhaust pipe 17, 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. Operation of the oxygen sensor constructed as aforesaid in conformity with the invention will now be described. When the exhaust gases flow from left to right in Fig. 1, 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 12 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 and 800°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, 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 kept at a high temperature of 800°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 a 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 is 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 la 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 automotive 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 embodiment of the invention shown in the Figs. enables to prevent 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 is capable of performing the operation of monitoring the oxygen concentration in the exhaust gases stably over a prolonged period of time. In the embodiment shown in Figs. 1-2, 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. By the invention, in oxygen sensors including all the types of sensor elements for monitoring the concentration of oxygen, can the following effects can be achieved: 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 adapted to be installed in a wall of a passageway (17) for gases to be tested, comprising: a sensor element (2); a base (1a) for attaching to a mount (18) of said wall, said base supporting electrically connecting terminals at one end portion thereof, the opposite end portion being exposed to a flow of gases in and through said passageway (17); and a cup-shaped protective member (12) having a peripheral wall with gas inlet ports and gas outlet ports (12a-d) provided therein in the vicinity of said one end portion of the base (1a); characterized in that the sensor element (2) is supported by said opposite end portion of the base (1a) in a position remote from said gas ports (12a-d) in a direction transverse to the direction of said flow of gases in and through said passageway (17), so that only a part of the gas entering through the gas ports flows along the base (1a) towards the sensor element (2); said opposite end portion of the base (1a) is enclosed by said protective member (12); said gas ports (12a-d) are arranged so as to come into immediate vicinity of the surface of the passageway (17). An oxygen sensor according to claim 1, characterized in that another gas port (12e) is formed at the protective member (12) in a position near said sensor element (2). A sensor element according to claim 2, characterized in that said other gas port (12e) has a smaller opening area than the gas inlet ports and gas outlet ports (12a-d), so that the former offers an essentially greater resistance to the flow of gases than the latter. An oxygen sensor according to anyone of claims 1 to 3, characterized in that the heat generating element is located in the immediate vicinity of the sensor element (2).

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