Structure of gas sensor ensuring high degree of gas-tight seal

An improved structure of a gas sensor is provided which is designed for achieving desired crimping of an end portion of a sensor housing to establish a higher degree of gas-tight seal between the housing and a sensor element. The sensor element is fitted within the sensor housing. The end portion of the sensor housing is crimped or bent to urge the sensor element into constant abutment with an inner wall of the housing through a sealing member. The housing has an unique shape and dimensions selected to ensure the higher degree of gas-tight seal regardless of the degree of wear of a crimper and/or dimensional error of the housing.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates generally to a gas sensor which is installed, for example, in an exhaust system of automotive internal combustion engines to measure a specified component of exhaust emissions, and more particularly to an improved structure of such a gas sensor which is designed to ensure desired crimping of a sensor housing to establish a higher degree of gas-tight seal between the housing and a sensor element.

2. Background Art

Typical gas sensors installed in an exhaust system (e.g., an exhaust manifold or exhaust pipe) of automotive internal combustion engines are constructed to have a sensor element fitted hermetically within a hollow cylindrical housing. Such fitting is achieved by crimping or bending an open end portion of the housing to bring the sensor element into constant abutment with an inner wall of the housing. The sensor element has formed therein an inner chamber used as a reference gas chamber into which air is admitted as a reference gas. An outer and an inner electrode are affixed to an outer and inner wall of the sensor element. The inner electrode is exposed to the inner chamber of the sensor element, while the outer electrode is exposed to a measurement gas chamber defined around the sensor element to measure the concentration of a specified component of exhaust gas of the engine flowing into the measurement gas chamber. The crimping of the open end portion of the housing also establishes a gas-tight seal between the sensor element and the housing, that is, between the measurement gas chamber and the reference gas chamber.

U.S. Pat. No. 6,303,013 B1 to Watanabe et al., assigned to the same assignee as that of this application, teaches installation of a sensor element within a housing using the crimping techniques, as described above. The housing, as disclosed in Watanabe et al., is constructed to have an annular extension to be crimped. The annular extension has a wall tapering toward an open end of the housing and dimensions selected to avoid bulging of the annular extension after being crimped.

The tapered wall of the annular extension has a maximum thickness which is more than twice a minimum thickness thereof. Specifically, the annular extension has greatly varying thickness, thus resulting in a difficulty in deforming the annular extension uniformly. Particularly, use of a greatly worn crimper or dimensional errors in the annular extension results in a difficulty to bend the annular extension to 90°, which usually leads to a lack of adhesion between the housing and the sensor element, that is, gas-tight seal between the measurement gas chamber and the reference gas chamber.

SUMMARY OF THE INVENTION

It is therefore a principal object of the invention to avoid the disadvantages of the prior art.

It is another object of the invention to provide an improved structure of a gas sensor which is designed to achieve desired crimping of a housing in order to ensure a higher degree of gas-tight seal between a measurement gas chamber and a reference gas chamber defined within the gas sensor.

According to one aspect of the invention, there is provided an improved structure of a gas sensor working to measure a given component content in a gas. The gas sensor comprises: (a) a hollow cylindrical housing having a length with a first and a second end portion opposed to each other; (b) a sensor element fitted within the hollow cylindrical housing, the sensor element having formed therein a reference gas chamber; (c) a measurement gas-exposed cover joined to the first end portion of the hollow cylindrical housing, the measurement gas-exposed cover having defined therein a measurement gas chamber into which a gas to be measured by the sensor element is admitted and to which the sensor element is exposed; (d) a reference gas-exposed cover joined to the second end portion of the hollow cylindrical housing, the reference gas-exposed cover having defined therein a reference gas chamber into which a reference gas is admitted and which leads to the reference gas chamber of the sensor element; (e) a sealing member disposed between the first end portion of the hollow cylindrical housing and the sensor element; and (f) an annular end portion formed at the first end portion of the hollow cylindrical housing. The annular end portion includes an annular neck and an annular extension which extends from the annular neck toward a tip end of the first end portion and is greater in outer diameter than the annular neck. The annular extension is crimped to urge the sensor element into constant abutment with the hollow cylindrical housing through the sealing member to establish a gas-tight seal between the reference gas chamber of the reference gas-exposed cover and the measurement gas chamber. The annular extension before being crimped has a shape including an annular tapered portion having an outer diameter decreasing toward the tip end of the first end portion and an annular straight portion extending straight from the annular tapered portion toward the tip end of the first end portion. The annular straight portion is bent inwardly of the hollow cylindrical housing to urge the sensor element into constant abutment with the hollow cylindrical housing.

The above structure of the housing allows the straight portion to be bent to approximately 90° with a uniform degree of deformation thereof. This causes the sealing member to be pressed tightly in a longitudinal direction of the housing, thereby establishing tight adhesion between the sensor element and the housing. This ensures a higher degree of the gas-tight seal between the reference gas chamber of the reference gas-exposed cover and the measurement gas chamber. The uniform degree of deformation of the straight portion is achieved even when a many time-used crimper that is worn greatly or the annular end portion of the housing has dimensional errors.

In the preferred mode of the invention, if a thickness of a tip portion of the straight portion of the annular extension is defined as t1, a maximum thickness of the annular tapered portion is defined as t3, and a thickness of the annular neck is defined as t4in the annular end portion of the hollow cylindrical housing before the annular extension is crimped, a relation of t1<t4<t3may be met.

Specifically, when the thickness t1is smaller than the thickness t4, it allows the straight portion to be deformed with little deformation of the annular neck. When the thickness t4is smaller than the thickness t3, it facilitates ease of buckling the annular neck.

If a thickness of a base portion continuing the annular tapered portion is defined as t2in the annular end portion of the hollow cylindrical housing before the annular extension is crimped, a relation of (t1+t2)/2<t4may be met. This avoid bulging of the annular neck after the annular straight portion is crimped.

A relation of t1≦t2≦1.1×t1may also be met. This allows the annular straight portion to have the outer diameter uniform in the longitudinal direction of the housing or to be tapered toward the tip thereof. This facilitates ease of crimping the annular straight portion.

If a length of the annular extension oriented to a longitudinal direction of the hollow cylindrical housing is defined as L1, and a length of the straight portion oriented to the longitudinal direction of the hollow cylindrical housing is defined as L2, a relation of 0.4×L1<L2<0.7×L1may be met. This facilitates ease of crimping the annular straight portion.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawings, wherein like reference numbers refer to like parts in several views, particularly toFIG. 2, there is shown a gas sensor1according to the invention which is designed to be installed in an exhaust system of an automotive internal combustion engine to measure an oxygen content in exhaust gasses for burning control of the engine. Note that the present invention is not limited to an oxygen sensor and may alternatively be used with a variety of gas sensors such as HC, CO, and NOx sensors.

The gas sensor1generally includes a sensing element3, a hollow cylindrical housing2, a measurement gas-exposed cover assembly5, and an air-exposed cover6. The housing2has formed therein an inner chamber201which is open at upper and lower ends, as viewed in the drawing. The sensor element3is retained within the inner chamber201of the housing2. The measurement gas-exposed cover assembly5is joined at an end thereof to the lower end of the housing2. The air-exposed cover6is joined to the upper end of the housing2. The measure gas cover assembly5, the housing2, and the air-exposed cover6are aligned in a longitudinal direction L to define a length of the gas sensor1.

The sensing element3is made of a cup-shaped solid electrolyte body which defines therein a reference gas chamber30into which air is admitted as a reference gas. The measurement gas-exposed cover assembly5has defined therein a gas chamber50within which a top portion (i.e., a sensing portion) of the sensor element3is exposed to gas to be measured. The air-exposed cover6has defined therein a reference gas chamber60leading to the reference gas chamber30of the sensor element3.

The housing2has an annular end portion20formed at the upper end thereof. The annular end portion20is crimped or curled inwardly of the housing2to retain the sensor element1within the housing2firmly.FIG. 1is an enlarged sectional view which shows the annular end portion20before being curled inwardly. Sealing parts4are disposed between the annular end portion20and the sensor element3. The firm installation of the sensor element3in the housing2is achieved by pressing and bending the annular end portion20inwardly in abutment with the sealing parts4to secure the sensor element3within the housing2.

The annular end portion20, as clearly shown inFIG. 2, is made up of an annular neck22and an annular crimp extension21continuing from the annular neck22to have a tip end211A.

Before being bent to achieve the above crimp installation, the crimp extension21is, as clearly shown inFIG. 1, made up of an annular tapered section212and an annular straight section211. The tapered section212has an outer wall tapering toward the tip end211A. The straight section211is substantially uniform in diameter and continues from the tapered section212. The crimp installation is, as can be seen fromFIGS. 2 and 7, achieved by bending or plastic-deforming the straight section211inwardly to fit the sensor element3within the housing2firmly.

The gas sensor1, as referred to herein, is an oxygen (O2) sensor which is installed in the exhaust system of the automotive internal combustion engine for use in burning control thereof.

The sensor element2is, as described above, of a cup-shape which includes a solid electrolyte body having the reference gas chamber30formed therein. The operation and structure of the sensor element2are well known in the art and not a major part of this invention, and explanation thereof in detail will be omitted here.

The sensor element2has a length extending in the longitudinal direction L and has a closed top end. The solid electrolyte body has an outer and an inner electrode affixed to an outer and an inner wall thereof. Within the reference gas chamber30, a bar-shaped heater35is disposed which works to heat the solid electrolyte body of the sensor element3up to a desired activation temperature at which the concentration of oxygen can be measured correctly.

The outer and inner electrodes of the sensor element3are electrically connected to sensor output lines301. The sensor output lines301are electrically joined to leads131within a porcelain insulator11. The leads131extend outside the gas sensor1through a bush12.

The sealing parts4are, as clearly shown inFIG. 2, a powder seal43made of talc etc., an insulator42, a metal ring41, and a metal gasket44. The insulator42works to insulate the sensor element3from the housing2. The metal ring41is disposed between the annular crimp extension21and the insulator42in abutment therewith to achieve a hermetical seal therebetween. The metal gasket44is disposed between an outer annular tapered shoulder31of the sensor element3and an inner annular tapered shoulder24of the housing2to enhance adhesion therebetween. The powder seal43, the insulator42, and the metal ring41are disposed within a cylindrical chamber25defined between an outer periphery of the sensor element2and an inner periphery of the housing2.

Specifically, the metal gasket44, the tapered shoulder31of the sensor element3, the powder seal43, the insulator42, and the metal ring44are retained in firm abutment with each other between the tapered shoulder24of the housing2and the crimp extension21of the annular end portion20of the housing2under an elastic pressure produced by crimping the extension21inwardly of the housing2.

The air-exposed cover6is welded at an open end portion601thereof to the housing2and surrounds the annular end portion20of the housing2and is exposed to air during use of the gas sensor1. The porcelain insulator11is disposed within the air-exposed cover6. The rubber bush12is fitted in an open end portion602of the air-exposed cover6which is opposed to the open end portion601in the longitudinal direction L.

The air-exposed cover6has formed therein air vents63which lead to the reference gas chamber60for inducting the air thereinto as the reference gas. A cylindrical water-repellent filter61is disposed around the air vents63. An outer cover62is affixed to a small-diameter portion of the air-exposed cover6. Such affixing is achieved by crimping the outer cover62, thereby also retaining the filter61between the outer cover62and the air-exposed cover6. The outer cover62also has air vents64communicating with the air vent63through the filter61.

The air as used as the reference gas in the sensor element3enters the air vents63and64from outside the gas sensor1and flows into the reference gas chamber30in the sensor element3through the reference gas chamber60in the air-exposed cover6.

The measurement gas-exposed cover assembly5is, as described above, installed at an end thereof in an annular groove formed in the bottom of the housing2. The measurement gas-exposed cover assembly5is made up of an inner cover51and an outer cover52both of which have gas inlets53through which the measurement gas is admitted into the gas chamber50to which the sensing element3is exposed.

The housing2has, as shown inFIGS. 1 and 2, a cover weld portion23to which the air-exposed cover6is welded. The cover weld portion23is disposed within the open end portion601of the air-exposed cover6. The cover weld portion23has an outer diameter smaller than a maximum outer diameter of the housing2. The cover weld portion23is formed between the annular end portion20and a flange70of the housing2.

The straight section211of the crimp extension21is, as described above, substantially uniform in diameter. Similarly, the cover weld portion23is substantially uniform in diameter.

The crimping of the annular crimp extension21is accomplished with cold crimping and hot crimping. The cold crimping is achieved by pressing the annular straight section211vertically (i.e., in the longitudinal direction L) to bend it using a cold crimper at a room temperature. The hold crimping is achieved after the cold crimping by placing a hot crimper in abutment with the bent annular straight section211, heating and softening the annular straight section, and pressing the annular straight section211to deform it further. We have evaluated a crimping condition, as will be discussed below, of the annular crimp extension21after being bent by the cold crimping.

The crimping condition is, as shown inFIG. 7, expressed by a parameter of an angle θ which a line A1extending through the center of the metal ring41in the longitudinal direction L makes with a line A2extending through the center of the metal ring41and a contact between an outer surface of the metal ring41and an inner surface of the annular straight section211of the annular crimp extension21. The angle θ will also be referred to as a crimp angle below. We have concluded that more near the crimp angle θ is to zero (0°)., the more excellent the crimping condition is.

The crimp angle θ of 0° is achieved when the annular straight section211has been bent to approximately 90°. When the crimp angle θ of 0° is achieved, it results in a maximum pressure acting on the powder seal43.

The shape of the annular crimp extension21of the housing2before being crimped is specially designed to enhance the degree of gas-tight sealing between the housing2and the sensor element3. Specifically, the annular crimp extension21has dimensions t1to t4and L1and L2, as discussed below.

Referring back toFIG. 1, the dimension t1is the thickness of the tip end211A of the annular straight section211, that is, the distance between the inner surface of an open end portion of the straight section211exposed to the inner chamber201and the outer surface of the open end portion of the straight section211. The dimension t2is the thickness of a base portion211B of the straight section211, that is, the distance between the inner and outer surface of the annular crimp extension21at an interface between the straight section211and the tapered section212. The dimension t3is the maximum thickness of the tapered section212, that is, the maximum distance between the inner surface of the tapered section212exposed to the inner chamber201and the outer surface of the tapered section212. The dimension t4is the thickness of the annular neck22, that is, the distance between the inner surface of the annular neck22exposed to the inner chamber201and the outer surface of the annular neck22.

The dimension L1is the length of the annular crimp extension21in the longitudinal direction L of the housing2, that is, the distance between the end surface of the tip end211A and an interface between the tapered section and the annular neck22. The dimension L2is the length of the straight section211, that is, the distance between the end surface of the tip end211A and the interface between the straight section211and the tapered section212.

The dimensions t1to t4and L1and L2of the annular crimp extension21have relations of t1<t4<t3, (t1+t2)/2<t4, t1≦t2≦1.1×t1, and 0.4×L1<L2<0.7×L1. For example, the dimensions t1and t2are equal to each other and are approximately 0.9 mm. The dimension t3is approximately 1.3 mm. The dimension t4is approximately 11.0 mm. The dimension L1is approximately 3.7 mm. The dimension L2is approximately 1.8 mm.

The reason for the above dimensional relations will be discussed below.

The relation of t1<t4has been derived by the experimental fact that when the thickness t1of the tip end211A of the straight section211is smaller than the thickness t4of the annular neck22, it allows the straight section211to be bent by the cold crimping with little deformation of the annular neck22. The relation of t4<t3has been derived by the experimental fact that when the thickness t4of the annular neck22is smaller than the maximum thickness t3of the tapered section212, it allows the annular neck22to be buckled by the hot crimping in the longitudinal direction L with little buckle of the tapered section212in the longitudinal direction L.

We performed first to third tests supporting evidences for deriving the relations (t1+t2)/2<t4, t1≦t2≦1.1×t1, and 0.4×L1<L2<0.7×L1. Results of the tests are shown inFIGS. 3 to 5.

FIG. 3is a graph which represents the results of the first test as performed to derive the evidence for the relation of (t1+t2)/2<t4. The abscissa axis indicates the value of t4/((t1+t2)/2) The ordinate axis indicates a change (mm) in outer diameter φD of the annular neck22of the housing2.

We prepared six samples of the housing2which had values of t4/((t1+t2)/2) between 0.9 to 1.3, subjected the samples to the cold crimping, and measured changes in outer diameters φD of the annular necks22of the samples. Note that a crimper after 10000 uses was used in the first test in view of wear thereof which usually arises after a lot of uses. Each of the samples was designed to have relations of t2/t1=1 and t3/t1=1.5.

The graph shows that when the value of t4/((t1+t2)/2) is between 1.0 to 1.3, the outer diameter φ D of the annular necks22of each of the samples hardly changes, while when the value of t4/((t1+t2)/2) is between 0.9 to 0.95, the outer diameter φD of the annular necks22of each of the samples increases. In other words, it is found that when an average thickness of the annular straight section211(i.e., (t1+t2)/2) is less than or equal to the thickness t4of the annular neck22, the outer diameter φD of the annular necks22of each of the samples hardly changes. For this reason, the housing2of this embodiment is designed to have the relation of (t1+t2)/2<t4.

FIG. 4is a graph which represents the results of the second test as performed to derive the evidence for the relation of t1≦t2≦1.1×t1. The abscissa axis indicates the value of t2/t1. The ordinate axis indicates the crimp angle θ of the annular crimp extension21, as illustrated inFIG. 7.

We prepared four samples of the housing2having values of t2/t1between 0.9 to 1.3, subjected the samples to the cold crimping, and measured the crimp angle θ (°) of the annular crimp extension21. Note that a crimper after 10000 uses was used in the second test in view of wear thereof which usually arises after a lot of uses. Each of the samples was designed to have relations of L2/L1=0.5 and t3/t1=1.5.

The graph shows that when the value of t2/t1is 1 or 1.1, it permits the crimp angle θ of the annular crimp extension21to be decreased below 13°, while when the value of t2/t1is 1.3 or 1.5, it causes the crimp angle θ to be increased above 15°. In other words, it is found that when the value of t2/t1is between 1 and 1.1, it permits the crimp angle θ of the annular crimp extension21to be decreased desirably. For this reason, the housing2of this embodiment is designed to have the relation of t1≦t2≦1.1×t1.

We also performed an additional test using an unused crimper. Other test conditions are identical with those in the second test. Results of the test show that it is possible to decrease the crimp angle θ of the annular crimp extension21below approximately 6° using the unused crimper. We have also found that when the value of t2/t1is less than 1, it causes the annular straight section211to increase in diameter toward the tip end211A and the housing2having such a dimension is unsuitable for the cold crimping required in this embodiment.

FIG. 5is a graph which represents the results of the third test as performed to derive the evidence for the relation of 0.4×L1<L2<0.7×L1. The abscissa axis indicates the value of L2/L1. The ordinate axis indicates the crimp angle θ of the annular crimp extension21, as illustrated inFIG. 7.

We prepared seven samples of the housing2having values of L2/L1between 0 to 0.9, subjected the samples to the cold crimping, and measured the crimp angle θ (°) of the crimp extension21. Note that a crimper after 10000 uses was used in the second test in view of wear thereof which usually arises after a lot of uses. Each of the samples was designed to have relations of t2/t1=1 and t3/t1=1.5.

The graph shows that when the value of L2/L1is between 0.4 and 0.9, it permits the crimp angle θ of the annular crimp extension21to be decreased below 10°, while when the value of L2/L1is 0 or 0.2, it causes the crimp angle θ to be increased above 20°. In other words, it is found that when the value of L2/L1is greater than or equal to 0.4, it permits the crimp angle θ of the annular crimp extension21to be decreased desirably. We have also found that when the value of L2/L1is greater than or equal to 0.7, it results in an undesirabley increase in the length L2of the annular straight section211relative to the length L1of the annular crimp extension21, thereby causing the hot crimping following the cold crimping to heat the annular straight section211up to an undesirable temperature, resulting in a difficulty in buckling the annular neck22desirably, which leads to a lack of stress oriented in the longitudinal direction L which is required to ensure constant adhesion between the housing2and the sensor element3. For this reason, the housing2of this embodiment is designed to have the relation of 0.4×L1<L2<0.7×L1.

The manner in which the straight section211of the annular crimp extension21of the annular end portion20is bent or crimped inwardly of the housing2to retain the sensor element3within the housing2firmly will be discussed below.

Prior to crimping the straight section211, the metal gasket44is, as shown inFIG. 2, placed on the annular tapered shoulder24of the housing2, after which the sensor element3is inserted into the inner chamber201of the housing2. Subsequently, powder such as talc is loaded into the cylindrical chamber25between the outer wall of the sensor element3and the inner wall of the housing2and compressed using a press to form the powder seal43.

The insulator43and the metal ring41are placed on the powder seal43.

Next, the annular crimp extension21of the housing2is bent inwardly by two steps: the cold crimping and the hot crimping. The cold crimping is accomplished at a room temperature using a ring-shaped crimper71, as illustrated inFIG. 6. The hot crimping is accomplished by pressing the annular crimp extension21and heating the annular neck22to soften it, thus causing the annular neck22to buckle.

The ring-shaped crimper71has an inner surface711curved into a shape matching with an expected outer surface of the straight section211after being bent by the cold crimping. After the metal gasket44, the powder seal43, the insulator42, and the metal ring41are arrayed within the cylindrical chamber25, the crimper71is brought close to the housing2from the longitudinal direction L until it abuts the tip end211A of the annular straight section211of the annular crimp extension21.

Next, the crimper71is forced downward, as viewed inFIG. 7, to press or bend the annular straight section211into a shape contoured to conform with the shape of the inner surface711of the crimper71, thereby wrapping the metal ring41with the straight section211. The bending of the straight section211results in slight deformation of the annular tapered section212. The straight section211is, as described above, uniform in thickness, so that it undergoes substantially uniform deformation as a whole. In this way, the sensor element3is retained firmly within the housing2through the metal gasket44, the powder seal43, the insulator42, and the metal ring41.

Finally, the hot crimping is performed by supplying current to the annular end portion20to heat it and pressing the straight section211further in the longitudinal direction L using a hot crimper (not shown) into a shape contoured to conform with a shape of an inner surface of the hot crimper. This causes the annular neck22to buckle, thereby compressing the powder seal43further to enhance the adhesion or gas-tight sealing between the housing2and the sensor element3through the metal gasket44. The cold and hot crimping causes the annular crimp extension21to produce a great stress acting on the sealing parts4in the longitudinal direction L, thereby ensuring the firm adhesion between the housing2and the sensor element3.

As apparent from the above discussion, the cold crimping in this embodiment enables the straight section211to be crimped inwardly through the crimper71and subjected to substantially uniform deformation. The straight section211is bent to substantially 90°. The crimp angle θ of the annular crimp extension21is minimized. The substantially 90° bend deformation of the annular straight section211produces tight compression of the powder seal43in the longitudinal direction L, thereby establishing the firm adhesion between the housing2and the sensor element3while keeping the electrical insulation of the sensor element3from the housing2. This ensures the gas-tight seal between the gas chamber50in the measurement gas-exposed cover assembly5and the reference gas chamber60within the sensor element3. Specifically, when the measurement gas enters the gas chamber50, and the air or the reference gas enters the reference gas chambers60and30, the gas-tight seal between the housing2and the gas sensor3provided by the sealing parts4works to isolate the measurement gas from the reference gas completely regardless of a rise in temperature of the measurement gas (i.e., the exhaust gas of the engine), thus enhancing the accuracy of measuring the concentration of the measurement gas (i.e., O2).

Even when the many time used cold crimper71is used to crimp the annular crimp extension21of the housing2or when the annular crimp extension21has any dimensional error, the cold and hot crimping in this embodiment enables the straight section211to be deformed substantially uniformly and the crimp angle θ of the straight section211to be kept minimized.

We have found that after the cold and hot crimping, the annular neck22hardly change in the outer diameter φD, and the cover weld portion23is also hardly deformed. It is, therefore, easy to insert the cover weld portion23into the open end portion601of the air-exposed cover6, thus facilitating ease of assembling of the housing2and the air-exposed cover6.

We performed a crimping test, as shown inFIG. 8, using the crimper71after used 10000 times to subject a crimp extension91of a housing, as used in a conventional gas sensor, which tapers toward an end tip911to the cold crimping. We found, as illustrated inFIG. 9, that it is difficult to bent the crimp extension91to 90° and decrease the crimp angle θ to a desired small value and that the structure of the housing2of the gas sensor1of this embodiment ensures the desired crimping of the crimp extension21, thus establishing the higher degree of gas-tight seal between the housing2and the sensor element3.