Gas sensor

A gas sensor is provided which can efficiently flow air into a casing while having a sufficient on-axis sectional area in a ceramic separator for allowing lead wires to pass therethrough. The oxygen sensor has a ceramic separator main body portion axially formed with a plurality of separator-side lead wire insertion holes penetrating therethrough, whereby a first part thereof is arranged to extend to an inner side of a filter support portion. The first part has an on-axis sectional outer shape formed in a shape other than a circle, e.g. a polygonal shape, such as a square shape. By making the on-axis sectional outer shape of the ceramic separator main body part non-circular as above, it is possible to locally expand a gap defined between an outer peripheral surface of the ceramic separator and an inner surface of the filter support portion. This makes it possible to efficiently flow external air into the casing while providing the ceramic separator with a sufficient on-axis sectional area for passing through lead wires.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to a gas sensor, such as an oxygen sensor, HC sensor,
 NOx sensor, etc., which measures a gas introduced into the sensor and
 detects a component of the gas.
 2. Description of the Related Art
 Conventional gas sensors have a metal casing, inside which resides a
 detection element formed in a bar-like or cylindrical shape and having, at
 a tip, a detecting section to detect a component gas. For a gas sensor
 using air as a reference gas, e.g. an oxygen sensor, it is necessary to
 provide a portion for introducing air into the casing and accommodating a
 detection element or providing a filter support cylinder coupled to a rear
 end of the casing. This type of air introducing element can be formed, for
 example, by opening a gas passage hole in a rear end side wall of the
 casing or filter support cylinder and covering it with a water repellent
 filter. Meanwhile, an air introducing element may be provided by a lead
 wire insertion hole formed in a rubber grommet sealing lead wire extension
 port. This method introduces air into a casing through a gap formed
 between a coat material covering a lead-wire outer surface and a lead wire
 insertion hole.
 In some conventional structures as described above, a ceramic separator
 formed with a lead wire insertion hole is provided in a casing (or filter
 support cylinder) in order to prevent a short circuit between lead wires
 extending from a detection element and heater. This ceramic separator is
 formed in a cylindrical shape matched to the casing, and is usually
 arranged at a rear end inside of the casing (or filter support cylinder)
 in order to be positioned on a side to extend a lead wire therefrom. On
 the other hand, an air introducing element as above is formed at the
 casing (or filter support cylinder) rear end, and a gap is provided
 between an outer peripheral surface and an inner peripheral surface of the
 casing (or filter support cylinder), in many cases, to provide a passage
 for the introduced air. However, recently there has been a movement toward
 reducing the size of the sensor, and, thus, there is an effort to reduce
 the diameter of the casing (or filter support cylinder) as small as
 possible. Thus, due to this effort to reduce sensor size, it is becoming
 difficult to provide a sufficient gap between the ceramic separator and
 the casing (or filter support cylinder). Although it is theoretically
 possible to reduce the diameter of the ceramic separator, there is a
 limitation on the ability to reduce the diameter of the ceramic separator
 size reduction on the assumption of securing and still maintain the basic
 function of the ceramic separator and prevent against shorting between the
 lead wires.
 It is therefore an object of the invention to provide a gas sensor which
 can efficiently flow air through the casing while having a sufficient
 on-axis sectional area for a ceramic separator that allows lead wires to
 pass through.
 SUMMARY OF THE INVENTION
 In order to solve the problems described above, the invention provides a
 first structure of a gas sensor that includes a detection element formed
 in an axial form, a cylindrical casing for accommodating the detection
 element, and a ceramic separator in a columnar form placed in an axial
 direction of the casing and having a plurality of lead wire insertion
 holes formed penetrating therethrough in the axial direction to pass
 through each lead wire from the detection element, wherein the ceramic
 separator depicts inscribed and circumscribed circles about a geometric
 center of gravity of an external shape line on an arbitrary section plane
 (hereinafter referred to as a contour forming on-axis section plane)
 perpendicular to the axis and on which an external shape of the ceramic
 separator can be represented in a continuous single contour, wherein, if
 depicting a virtual reference circle positioned intermediate between the
 inscribed circle and the circumscribed circle, the contour forming on-axis
 section plane having an external shape line that in one par is positioned
 on an inner side of the reference circle and in other part on an outer
 side of the reference circle.
 The structure described above essentially provides a ceramic separator
 having an outer shape line on a contour forming on-axis section plane that
 at least partly departs from a circular shape. FIG. 8 shows an example of
 a case where, for example, the main body portion has an outer shape line
 SH rendered in a squared form on the contour forming on-axis section
 plane. With respect to an outer shape SH, inscribed and circumscribed
 circles IC and OC are depicted about a geometric center of gravity, and a
 reference circle SC is depicted between these circles. It can be
 understood that the outer shape line SH is at some points within the
 reference circle SC and at other points outside the reference circle SC.
 This would be true for a shape that departs from a circular shape
 (polygonal, elliptical, etc.) and is not limited to only a squared shape
 like outer shapeline SH.
 That is, in a conventional gas sensor structure, the ceramic separator,
 without exception, is circular in shape on a contour forming an on-axis
 section plane. As described earlier, there has been difficulty in
 expanding the gap defined with a casing or filter support cylinder inner
 surface. According to the invention, however, the external shape line of
 the ceramic separator on a contour forming an on-axis sectional plane is
 constructed as to depart from the traditional circular shape. This makes
 it possible to locally expand the gap between the outer peripheral surface
 of the ceramic separator and the casing or filter support cylinder inner
 surface. Due to this, passage of external air into the casing occurs
 efficiently while providing the ceramic separator with a sufficient
 on-axis sectional area for inserting through lead wires.
 Incidentally, where the ceramic separator has, for example, a main body
 portion in a columnar shape and a flanged separator side support portion
 formed projected on an outer peripheral surface of the main body, the
 external shape line on the contour forming on-axis section plane may be at
 least one of an external shape line of the main body portion and an
 external shape line of the separator side support portion.
 The invention also provides another gas sensor structure that includes a
 detection element formed in an axial form, a cylindrical casing for
 accommodating the detection element, a filter support cylinder provided
 generally coaxial to the casing over an rear end portion of the casing, a
 ceramic separator in an columnar form placed in an axial direction of the
 filter support cylinder and having a plurality of lead wire insertion
 holes formed penetrating therethrough in the axial direction to pass
 through each lead wire from the detection element, a flanged
 separator-side support portion formed in an outer peripheral surface of
 the ceramic separator, a filter arranged on a rear side of the
 separator-side support portion and on an rear end side of the filter
 support cylinder, which blocks a liquid from passing through but allows a
 gas to pass therethrough, and a gas communication portion formed in the
 separator-side support portion in the axial direction.
 Incidentally, in the description of the invention above, the axial side
 toward the tip of the detection element is the "front side (tip side)",
 while the side opposite to this is a "rear side (rear end side)".
 In this manner, a flanged separator side support portion is formed on an
 outer peripheral surface of the ceramic separator. A filter is arranged on
 a rear side of this separator side support portion and on a rear end side
 of the filter support cylinder, which blocks a liquid from passing
 through, but allows a gas to pass. A gas communication portion is axially
 formed in the separator side support portion. This allows the air
 introduced through the filter to be smoothly and speedily introduced to an
 inside of the casing through the gas communication portion axially formed
 in the separator side support portion. Moreover, a filter is provided
 which blocks liquid from passing, but allows a gas to pass through.
 Accordingly, where mounted, for example, on a gas discharge pipe, or the
 like, close to a vehicular wheel, it is possible to introduce external air
 even if water is sprayed onto the structure during rain or during washing
 of the vehicle. Accordingly, the gas sensor can be reduced in outer
 diameter without impeding the performance of the sensor.
 The gas communication portion can form one part of a communication path for
 air directed from the filter to an internal tip side of the casing along
 the outer peripheral surface of the ceramic separator. In this manner,
 where the air introduced through the filter can be introduced to the
 internal tip side of the casing through a shortest communication path, the
 circulation of air as a reference gas is promoted to improve ventilation
 performance, enabling gas concentration detection with accuracy.
 Also, according to the invention, the separator side support portion can
 have an external contour line on the contour forming an on-axis section
 plane exhibiting a polygonal shape, and the gas communication portion
 being formed between an inner surface of the filter support cylinder and
 an outer peripheral surface of the separator side support portion in a
 form including the external shape line. In this case, the gap between the
 outer peripheral surface of the separator side support portion and the
 inner surface of the filter support cylinder can be locally expanded, thus
 securing a minimum amount of communication air to the inside of the
 casing. Also, because the external shape line on the contour forming
 on-axis section plane is in a polygonal form, it is possible to provide a
 homogeneous air flow state with respect to a circumferential direction.
 Furthermore, the gas communication portion of the invention can be formed
 in a flat plane or groove forming the outer peripheral surface, or in a
 pore form penetrating through the separator side support portion. In any
 case, the gas communication portion can be integrally formed during
 forming of the ceramic separator. Thus, manufacture can be accomplished
 easily and at low cost.
 With respect to the gas structures described above, the following features
 may also be provided. On the rear end side of the casing a cylindrical
 filter support portion may be provided having one or a plurality of gas
 introducing pores formed in a wall thereof. A filter is arranged for
 closing the gas introducing pores in the filter support portion to block
 liquid from passing but to allow a gas to pass through, whereby a gas
 introducing structure portion is formed to introduce air into the casing
 through the gas introducing pores and the filter. The invention also
 provides that the main body part of the ceramic separator is arranged at
 an axially rear side of the detection element and extends to an inner side
 of the filter support portion, wherein a gap formed between the main body
 part outer peripheral surface and the filter support portion inner
 peripheral surface forms one part of a communication passage for the air
 directed through the filter to the internal tip side of the casing.
 A further embodiment of the invention provides that a ceramic separator is
 formed with a flanged separator side support portion projecting from an
 outer peripheral surface of the main body part at an axially intermediate
 position whereby the main body part at the separator side support portion
 is abutted against an rear end face of the casing directly or indirectly
 through another member in a state that a portion axially forwardly
 positioned of the separator side support portion is inserted to a rear end
 inner side of the casing, while an axially rear portion is arranged so as
 to project to an outer side of the casing where the projected portion is
 covered by the filter support portion formed in a cylindrical form
 separately from the casing, and further including a gas communication
 portion axially formed in the separator side support portion to allow a
 gas to flow from the inner side of the filter support portion to an inner
 side of the casing.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Now embodiments of the invention will be described with reference to the
 drawings.
 FIG. 1 shows the internal structure of an oxygen sensor according to one
 embodiment of the gas sensor according to the invention. The oxygen sensor
 1 includes an oxygen detection element (detection element) 2 formed by a
 solid electrolytic member in a hollow cylindrical form having a closed end
 and a heating element 3. The oxygen detection element 2 is hollow and is
 formed of an oxygen ion conductive solid electrolyte based on zirconia or
 the like. A metal casing 10 is provided on an outer side of the oxygen
 detection element 2.
 The casing 10 has main hardware 9 formed with a threaded portion 9b to
 mount the oxygen sensor 1 on a mount part, such as an exhaust pipe, a main
 cylinder 14 coupled in internal communication to one opening of the main
 hardware 9, and a protector 11 attached to the main hardware 9 from a side
 opposite to the main cylinder 14. The oxygen detection element 2 has, as
 shown in FIG. 2, a pair of electrode layers 2b, 2c which are porous, e.g.
 of Pt or a Pt alloy, to cover over almost the entire inner and outer
 surfaces thereof.
 Referring back to FIG. 1, the main body hardware 9 at the opening on a rear
 side is crimped with the main cylinder 14 through a ring 15 on an
 insulator 6. The main cylinder 14 is fixed, by fitting it with a
 cylindrical filter assembly 16. The filter assembly 16 is sealed by a
 grommet 17 at its rear end opening. The grommet 17 is formed from rubber
 or the like. Inward of this, a ceramic separator 18 is provided. Lead
 wires 20, 21 for the oxygen detection element 2 as well as lead wires for
 the heating element 3 (not shown) are provided penetrating through the
 ceramic separator 18 and grommet 17.
 As shown in FIG. 3, the ceramic separator 18 has a plurality of
 separator-side lead insertion holes 72 (lead wire insertion hole) axially
 formed penetrated for passing through the lead wires 20, 21, and a flanged
 separator-side support portion 73 formed in an axially intermediate
 position thereof in a form projecting from an outer peripheral surface.
 The ceramic separator 18 is received at its forward portion of the
 separator-side support portion 73 in a rear end inner face of the main
 cylinder 14 so that it abuts against a rear end surface of the main
 cylinder 14. Further, a rear portion of the separator support portion 73
 is projected outward of the main cylinder 14.
 Referring back to FIG. 1, one lead wire 20 for oxygen detection element 2
 is electrically connected to the electrode layer 2c (FIG. 2) provided on
 the inner surface of the oxygen detection element 2 by way of an fixing
 hardware 23. On the other hand, the other lead wire 21 is electrically
 connected to the outer electrode layer 2b(FIG. 2) of the oxygen detection
 element 2 by way of another fixing hardware 33. The oxygen detection
 element 2 is activated due to heating up of the heating element 3 placed
 inside thereof. The heating element 3 is a rod-shaped ceramic heater, and
 has a heating portion 42 having a resistance heat generating part (not
 shown) to be energized through a lead wire (not shown) thereby heating a
 tip (detecting section) of the oxygen detection element 2.
 As shown in FIG. 3, the filter assembly 16 assumes a cylindrical shape
 axially coupled to the main cylinder 14 (casing 10) from a rear outer
 side, via filter support part 51; filter support part 51 (filter support
 cylinder) having an interior communicated with an outside of the main
 cylinder 14 and a wall formed with a plurality of gas introducing pores
 52. On an outer side of the filter support part 51, a cylindrical filter
 53 is provided to close the gas introducing pores 52. Further, on an outer
 side of the filter 53, an auxiliary filter support part 54 is provided
 which has a wall formed with one or a plurality of auxiliary gas
 introducing pores 55 and holds the filter 53 by sandwiching it with the
 filter support part 51. Specifically, the gas introducing pores 52 and
 auxiliary gas introducing pores 55 are circumferentially formed in each
 axially intermediate position at a predetermined interval in a
 corresponding positional relation to each other with respect to the filter
 support part 51 and auxiliary filter support part 54. The filter 53 is
 arranged in a manner circumferentially surrounding the filter support part
 51. Incidentally, the filter 53 is in a porous fibrous structure, for
 example, of polytetrafluoroethylene (product name: Goatex (Japan Goatex
 Co. Ltd.)), which is structured as a water-repellent filter to block a
 liquid based on water such as water droplets from passing through but
 allows a gas such as air and/or water vapor to pass through. Incidentally,
 both the main cylinder 14 and the filter support part 51 are cylindrical
 in shape. Incidentally, the filter 53 is in a porous fibrous structure,
 for example, of polytetrafluoroethylene (product name: Goatex (Japan
 Goatex Co. Ltd.)), which is structured as a water-repellent filter to
 block a liquid based on water such as water droplets from passing through
 but allows a gas such as air and/or water vapor to pass through.
 Incidentally, both the main cylinder 14 and the filter support part 51 are
 cylindrical in shape.
 As shown in FIG. 4, the auxiliary filter support part 54 has annular filter
 crimp portions 56, 57 (hereinafter referred merely as crimp portions 56,
 57) formed at positions sandwiching a row of the auxiliary gas introducing
 ports 55 and respective sides of their axes. By the crimp portions 56, 57,
 the auxiliary filter support part 54 is coupled to the filter support part
 51 through the filter 53. Also, a gap 58 is provided between an outer
 surface of the filter support part 51 and the filter 53. On the other
 hand, the filter support part 51 includes a step portion 60 formed in an
 axially intermediate portion to have a first portion 61 that is axially
 forwardly positioned with respect to the step portion 60 and a second
 portion 62 that is axially rearwardly positioned. The second portion 62 is
 formed smaller in diameter than the first portion 61. The gas introducing
 pores 52 are formed in a wall of the second portion 62. Further, the
 auxiliary filter support part 54 has an inner diameter smaller than an
 outer diameter of the first portion 61 of the filter support part 51.
 Referring back to FIG. 3, the filter support part 51 covers the projected
 portion of the ceramic separator 18 extended up to an inner side of the
 second portion 62, and is arranged to abut at the step portion 60 against
 the separator-side support portion 73 through a metal elastic member 74
 from an opposite side to the main cylinder 14. On the other hand, the
 filter support part 51 at a tip side, or at the first portion 61, is
 arranged to overlap, from an outer side, with the main cylinder 14 (casing
 10). The overlap portion has a casing crimp portion 76 to air-tightly
 couple the filter support part 51 to the main cylinder 14.
 The auxiliary filter support part 54 has, at its outer side, a cylindrical
 protect cover 64 in a manner covering over same. This protect cover 64, as
 show in FIG. 3, is arranged to provide a gas staying space 65, and joined
 by crimp portions 66, 67 to the filter support part 51. Incidentally, as
 shown in FIG. 4, a plurality of grooves 69 are circumferentially formed at
 a predetermined interval in an outer peripheral surface of the first part
 61, which serve as a gas introducing part to an inside of the protect
 cover 64.
 Referring back to FIG. 3, the ceramic separator 18 is placed such that it
 extends at its rear side to an inside of the filter support part 51 and at
 its front side to an inside of the main cylinder 14 (casing 10) with
 respect to an axial direction of the oxygen detection element 2. The leads
 20, 21, etc. are axially inserted in the separator-side lead wire
 insertion hole 72. On the other hand, grommet 17 is resiliently fitted in
 a rear-side opening 51a of the filter support part 51 to have a seal-side
 lead wire insertion hole 91 for inserting the lead wires 20, 21, etc.
 therein, and provides sealing between an outer surface of the lead wires
 20, 21, etc. and an inner surface of the filter support part 51.
 The rear end face of the ceramic separator 18 is positioned axially rear to
 the gas introducing ports 52. The rear end is centrally formed with a gap
 regulating projections 96 having summit surfaces that are close contacted
 with a front face of the grommet 17. Due to the gap regulating projections
 96, a predetermined amount of a gap 98 is given between the grommet 17 and
 the ceramic separator 18. Incidentally, the gap regulating projections 96
 may be formed in a front face of the grommet 17 instead of the ceramic
 separator 18. Also, a gap 92 is provided between an inner peripheral
 surface of the filter support part 51 and an outer peripheral surface of
 the ceramic separator 18. The gas from the gas introducing port 52 is
 supplied into the gap 92 and further introduced into the casing 10 via a
 gas introducing part 93 formed in the ceramic separator 18. Specifically,
 the ceramic separator 18 has an axial gas-passage through-hole 95 formed
 separately from the separator-side lead wire insertion holes 72, and a
 gas-passage groove 94 formed at its rear end face that has one end
 communicated with the through-hole 95 and the other end opened to the
 outer surface of the ceramic separator 18. That is, these gas-passage
 through-hole 95 and gas-passage groove 94 constitute a gas introducing
 part 93.
 Referring back to FIG. 1, the main hardware 9 is formed, at its front-side
 opening, with a cylindrical protector fitting portion 9a to which a
 cap-formed protector 11 is fitted, with a predetermined spacing, to cover
 a tip side (detecting section) of the oxygen detection element 2. The
 protector 11 is formed with a plurality of gas passing ports 12 to pass
 through an exhaust gas.
 In the oxygen sensor 1, air as a reference gas is introduced through the
 filter 53 of the auxiliary filter support part 54. On the other hand,
 exhaust gas introduced through the gas passing ports 12 of the protector
 11 comes into contact with an outer surface of the oxygen detection
 element 2. The oxygen detection element 2 has an oxygen concentration
 battery electromotive force dependent upon an oxygen concentration
 difference between inner and outer surfaces thereof This oxygen
 concentration battery electromotive force is taken out as a detection
 signal of an oxygen concentration in the exhaust gas from the electrode
 layer 2b, 2c (FIG. 2) through the lead wires 21, 20. Thus, an oxygen
 concentration in an exhaust gas can be detected.
 Incidentally, assembling a filter assembly 16 on the main cylinder 14 can
 be made, for example, as follows. That is, as shown in FIG. 14(a), a metal
 elastic member 74 is inserted in a ceramic separator 18 and further the
 ceramic separator 18 at its front side is inserted to a main cylinder 14.
 On the other hand, a filter assembly 16 having been previously assembled
 as shown in FIG. 4 is externally fitted at its filter support part 51 to
 the ceramic separator 18 and main cylinder 14, as shown in FIG. 14(a).
 Incidentally, an oxygen detection element 2, heating element 3, etc. (FIG.
 1) are previously assembled in the main cylinder 14. The lead wires 20,
 21, etc. of them are passed through the separator-side lead wire insertion
 holes 72 (FIG. 3) of the ceramic separator 18 and allowed to extend from a
 rear end opening of the filter support part 51.
 Subsequently, as shown in FIG. 14(b), the main cylinder 14 and filter
 assembly 16 are applied by utilizing an axial compression force. This
 causes the metal elastic member 74 to be compressed and deformed between
 the filter support member 51 and the separator-side support portion 73 of
 the ceramic separator 18, to cause an urging force for clamping the
 ceramic separator 18 between the main cylinder 14 and the filter support
 portion 51. While keeping this state, a casing crimp portion 76 is formed
 in the filter support part 51 and main cylinder 14, for joining them as
 shown in FIG. 14(c). Then, as shown in FIG. 14(d), the filter support
 portion 51 at its rear end opening is inserted by a rubber grommet 17 and
 further covered by a protect cover 64, and crimp portions 66 and 67 are
 formed as shown in FIG. 14(e), thus completing the assembling for the
 oxygen sensor 1.
 Next, as shown in FIG. 5(a), the ceramic separator 18 has a main part 75
 that is in a shape at least party different from a circular shape,
 specifically a square in on-axis sectional shape. In this embodiment, the
 main body part 75 (hereinafter, see also FIG. 3) comprises two parts, i.e.
 a part 18a to be inserted in the second section 62 of the filter support
 part 51 (hereinafter referred to as "first part") and a part 18b to be
 inserted to an inner side of the main cylinder 14 (hereinafter referred to
 as "second part"). The square columnar form in at least the first part 18a
 makes it possible to secure a relatively large gap 92 between the same and
 an inner surface of the filter support part 51, as shown in FIG. 7(a),
 thereby ensuring smooth air flow. Incidentally, the second part 18b of the
 main body part 75 may be formed in a circular form. Also, although in FIG.
 5(a) the gap regulating projections 96 are also formed in a square
 columnar form, this may be formed in a circular cylinder or other forms.
 Here, as shown in FIG. 3 the oxygen sensor 1 of the present embodiment has,
 on a rear end outer side of the casing 10, a cylindrical filter support
 portion 51 (filter support cylinder) formed with a gas introducing pores
 52 in a wall thereof and covered coaxially over the casing 10 from a rear
 side. The filter assembly 16 is arranged with a filter 53 to block a
 liquid from passing, but allows a gas to pass through, in a manner closing
 the gas introducing pores 52 of the filter support part 51. Thus, a gas
 introducing structure part is formed to introduce external air into the
 casing 10 through the gas introducing pores 52 and filter 53. Also, the
 main body part 75 of the separator 18 is arranged such that the first part
 18a on the rear side extends to an inside of the filter support part 51.
 Further, the gap 92 given between an outer peripheral surface of the first
 part 18a (main body part 75) and an inner peripheral surface of the filter
 support part 51 serves to form a part of a communication passage for
 external air to be introduced to an inside of the casing 10 through the
 filter 53. Because this structure allows external air to flow from the
 filter 53 side into the sufficiently broad gap 92, the external air flow
 within the casing 10 can be made with further smoothness.
 Also, the main body part 75 of the ceramic separator 18 at its rear end
 face is positioned rear of the gas introducing ports 52 provided in the
 filter support part 51, and the main body part 75 has the gas-passage
 through-hole 95 formed axially penetrating therethrough. In this case, the
 external gas introduced through the gas introducing pores 52 is to be
 guided to a tip portion of the detection element 2 through the gap 92. The
 flow path is as follows. That is, the introduced external air flows from
 the gap 92 to a rear side of the ceramic separator 18, and further through
 the gas-passage through-hole 95 to the internal tip side of the detection
 element 2. The polygonal shaped main body part 75 (its first part 18a) of
 the ceramic separator 18 realizes smooth gas passage through the gap 92.
 Here, as shown in FIG. 5(a) and FIG. 3 the ceramic separator 18 is formed
 with four separator-side lead wire insertion holes 72 which are arranged
 such that their centers are positioned on a hypothetical circle C1
 (separator-side pitch circle) in order to pass through the lead wires
 extended from the oxygen detection element 2 and heating element 3. Also,
 the gas-passage through-hole 95 is formed in a region surrounded by the
 four separator-side lead wire insertion holes 72 in a central part of the
 ceramic separator 18. Further, the gas-passage groove 94 is formed in a
 cross form in a position where there is no interference with the four
 separator-side lead wire insertion holes 72. The grommet 17 at its front
 end face contacts with the ceramic separator 18, in an opening position of
 the gas-passage through-hole 95. However, because the gas-passage groove
 94 is formed, the air is not prevented from flowing from the gap 92 to the
 gas-passage through-hole 95.
 Also, in FIG. 3, the grommet 17 is formed with a seal-side lead wire
 insertion hole 91 positioned on a seal-side pitch circle. The
 above-mentioned separator-side pitch circle (diameter D1) and the
 seal-side pitch circle (diameter D2) are set such that one is greater in
 diameter than the other. For example, in FIG. 3 a relationship D1&gt;D2 is
 given. As shown in FIG. 5(a), the gap regulating projections 96 are formed
 in an area positioned at inner side of the separator-side lead wire
 insertion holes 72 arranged on a separator-side pitch circle. In this
 case, although the lead wires are caused by bending between the grommet 17
 and the ceramic separator 18, a gap 98 is provided based on the gap
 regulating projection 96 between the grommet 17 and the ceramic separator
 18. Consequently, there is less possibility of causing such trouble that
 the lead wire be strongly bent resulting in damage or disconnection during
 assembling the sensor 1. Also, even if the grommet 17 is acted on by an
 axial pressing force, the grommet 17 is stopped from moving by the gap
 regulating projections 96. Thus, the gap amount is hardy changed and the
 lead wires are prevented from causing strong bending thereon.
 FIGS. 7(a), 7(b) and 7(c) show examples whereby the main body part 75 of
 the ceramic separator 18 is made in various polygonal shapes of an on-axis
 section. FIG. 7(a) is in a square columnar form as already shown, FIG.
 7(b) is in a triangular columnar form and FIG. 7(c) is in a octagonal
 column. As can be understood these figures, where the main body part 75 is
 made in a hexagonal form in on-axis shape, a circular shape is approached
 by increasing the number of sides. This, however, does not provide the
 significant effect of expanding the gap 92 between it and the filter
 support portion 51 (second part 62 thereof). On the other hand, as for the
 separator-side pitch circle, where the lead wires exist, for example, four
 in number, the triangular columnar form of FIG. 7(b) excessively reduces
 the pitch circle size. Accordingly, it can be said that the square
 columnar shape is preferred as a main body part 75 form for the ceramic
 separator 18 which can secure a sufficiently great pitch circle while
 securing a sufficient gap 92.
 In this case, if gas introducing pores 52 are provided corresponding to the
 on-section sides of the main body part 75, it is possible to efficiently
 introduce a gas to the gap 92 between the ceramic separator 18 and the
 filter support part 51. Also, if chamfer or round is given to each edge,
 such trouble as marring to the casing is relieved during assembling.
 FIGS. 9(a) and 9(b) show examples whereby the main body part 75 in an
 on-axis section is made in a form other than polygonal in shape. In FIG.
 9(a), cut-outs are oppositely provided with respect to an axis of the
 circular on-axis section thereby forming a pair of flat portions 75a, 75a
 that are parallel with each other. On the other hand, FIG. 9(b) is an
 example of an elliptic shape on the on-axis section.
 The ceramic separator 18 shown in FIG. 5(b) is not formed with a gas
 introducing groove 94 or gas introducing part 93 including gas-passage
 through-hole 95, etc. Also, a gap regulating projection 96 is formed at a
 center on a rear end face of the ceramic separator 18. In this case, as
 shown in FIG. 6, a gas-passage gap 99 (see FIG. 6) is circumferentially
 formed between the separator-side lead wire insertion hole 72 and the lead
 wires 20, 21, etc. to provide a structure that the air introduced through
 the gas introducing pores 52 is guided from the gap 72 through the
 gas-passage gap 99 to the detection element 2. This omits formation of a
 gas introducing part 93. Further, there is no necessity of forming a
 groove 94 or the like that extends between the gap regulating projections
 96 shown in FIG. 5(a), simplifying the shape and facilitating ease of
 manufacture.
 Incidentally, for the ceramic separator 18 shown in FIG. 5(b), gas-passage
 grooves 94 may be formed as shown in FIG. 5(c) that are directed from
 end-face outer peripheral edges thereof toward the respective
 separator-side lead wire insertion holes 72. This further makes the air
 flow to the detection element 2 (FIG. 1) within the casing more effective.
 A modification to the separator-side support portion 73 of the ceramic
 separator 18 will be shown in FIGS. 10(a) to 13 and FIG. 15. The ceramic
 separator 18 shown in FIG. 10 is an example of the outer peripheral
 surface of the flanged separator-side support portion 73 positioned inward
 of an opening inner edge 14a of the main cylinder 14 (casing 10), whereby
 retraction passages 80 (gas communication portion) are axially formed to
 partially eliminate a state of shielding an opening by the separator-side
 support portion 73 and allow air flow to an interior of the main cylinder
 14. The retraction passage 80 is formed in a flat surface form. Due to
 this, a comparatively large gap 81 is formed between an opening inner edge
 14a of the main cylinder 14 and an outer edge of the separator-side
 support portion 73, thus securing further smooth gas flow. The
 separator-side support portion 73 herein has, at its outer peripheral
 surface, a plurality of the retraction passages 80 formed in series in a
 circumferential direction, thus providing a polygonal exterior shape
 (square in the present embodiment). This enables a gas flow state with
 greater evenness with respect to a circumferential direction of the
 separator-side support portion 73.
 Incidentally, grooved retraction passages 83 (gas communication portion)
 may be formed in the outer peripheral surface of the separator-side
 support portion 73, as shown in FIG. 11. Also, FIG. 12 shows an example
 having air-passage holes 84 formed axially penetrating through the
 separator-side support portion 73, in place of the retraction passages. In
 any example, the retraction passages 83 or the air-passage holes 84 (gas
 communication portion) are formed in a plurality at a predetermined
 interval in the circumferential direction of the separator-side support
 portion 73. Incidentally, where the retraction passage or air-passage
 holes as above are formed in the separator-side support portion, the main
 body part 75 of the ceramic separator 18 may be structured without
 particularly forming axially-penetrating air passages (e.g. air-passage
 groove 94 or air-passage through-hole in FIG. 3 or air-passage gap 99 in
 FIG. 6). Further, the main body part 75 of FIG. 10(a) may be formed in a
 circular cylinder as shown in FIG. 13.
 The ceramic separator 18 shown in FIG. 15 is formed with flanged
 separator-side support portions 73 in a form projecting in one body
 fashion over the entire periphery at outer peripheral surface thereof on
 an axially rear side. The ceramic separator 18 has, at a position without
 interfering with four separator-side lead wire insertion holes 72, gas
 passage grooves 100 (gas communication portion) formed in a cross form in
 directions perpendicular to an axis on a rear end surface. Each gas
 passage groove 100 extends reaching the rear end outer periphery from
 which it changes in direction and extends toward an axially front side
 along an outer periphery of the separator-side support portion 73.
 FIG. 16 shows one example of an oxygen sensor incorporating the ceramic
 separator of FIG. 15. A cylindrical outer cylinder member 101 (filter
 support cylinder) at an axially front side is formed large in diameter and
 fitted over a main hardware 9 (casing 10). On the other hand, the outer
 cylinder member 101 at an axially rear side is formed small in diameter
 and accommodates therein the ceramic separator 18. A grommet 17 is fitted
 in a rear end opening. The outer cylinder member 101 has an outer
 cylinder-side support portion 102 receiving and supporting the
 separator-side support portion 73 of the ceramic separator 18. The grommet
 17 has a center through-hole 17b formed at the radial center. This center
 through-hole 17b receives a filter 53. The filter 53 has an air permeable
 surface provided in a rear end surface, and a cylindrical peripheral
 portion internally fitted with cylindrical filter support hardware 53a.
 In the oxygen sensor 1 of FIG. 16, air as a reference gas is introduced to
 an inner surface (internal electrode layer 2c) of the oxygen detection
 element 2 through the air permeable end surface.fwdarw.the air passage
 groove 73 of the ceramic separator 18.fwdarw.the radial gap S1, S2 between
 the outer cylinder member 101 and the ceramic separator 18.fwdarw.hollow
 portion 2a (see arrow R in FIG. 16). Incidentally, in FIG. 16, the parts
 common to those of FIG. 3 or 6 are denoted by the same reference
 characters, omitting explanation thereof.
 In the each case of FIGS. 10(a) to 13 and FIG. 15, the gas communication
 portion (retracted passage portion 83, air passage hole 84 or air passage
 groove 100) is provided in plurality along a circumferential direction at
 a predetermined interval and formed in an axial direction. Accordingly,
 this gas communication portion forms part of a passage for the air
 directed from the filter to an internal tip of the casing along the outer
 peripheral surface of the ceramic separator 18. Incidentally, where a gas
 communication portion is formed in the separator-side support portion 73
 as above, a gas communication portion (e.g. the air passage groove 94 or
 air-passage through-hole 95 in FIG. 3 or air passage gap 99 in FIG. 6) is
 not structured which axially penetrate through the main body part 75 of
 the ceramic separator 18 (e.g. see FIG. 15 and FIG. 16).
 The sensor structures as explained above are applicable similarly to the
 gas sensors other than the oxygen sensors, e.g. HC sensors or NOx sensors.
 While the invention has been described in conjunction with specific
 embodiments thereof, it is evident that many alternatives, modifications
 and variations will be apparent to those skilled in the art. Accordingly,
 the preferred embodiments of the invention set forth herein are intended
 to be illustrative, not limiting. Various changes may be made without
 departing from the spirit and scope of the invention as defined in the
 following claims.