Gas sensor

Abstract

Gas sensor (1, 2) comprising: a gas sensor element (10) extending in one direction along an axis (O) and having at its front end part a sensing part (11) for sensing a gas to be measured, a tubular metal shell (50) holding the gas sensor element (10) such that the sensing part (11) can project from its front end, and a protective element (100, 200) fixed to a front end part of the metal shell (50) and enclosing the sensing part (11), wherein the protective element (100, 200) comprises: an inner protective element (120) in the interior (129) of which the sensing part (11) of the gas sensor element (10) is received and which has a tubular side wall (122) with an inner gas inlet hole (130) for introducing the gas to be measured into the interior (129) and a bottom wall (124) at a front has an end of the side wall (122) and an outer protective element (110),the inner protective member (120) with a space (119) between the outer protective member (110) and the side wall (122) of the inner protective member (120), and which has a tubular side wall (112) with an outer gas inlet hole (115) provided in front of the inner gas inlet hole (130) with respect to the direction of the axis (O) and designed for introducing the gas to be measured into the space (119), wherein the outer protective member (110) has a truncated conical, tapered wall (117) tapering forward from a front end of the side wall (112) with respect to the direction of the axis (O), and an outer gas outlet hole (170) formed within a front end edge (117s) of the tapered wall (117) and a connection between the interior and exterior of the outer protective member (110) manufactures, comprises, wherein,where SL represents a surface defined by the front end edge (117s) of the tapered wall (117), the outer gas outlet hole (170) has a surface (S) satisfying the relational expression 1 / 2 × SL ≤ S ≤ SL, and wherein the bottom wall (124) of the inner guard member (120) has a bottom wall opening (160, 260), the inner guard member (120) having a cover (127, 227) that covers the bottom wall opening (160, 260) from a rear side with respect to the direction of the axis (O) and is recessed behind the bottom wall (124) with respect to the direction of the axis (O), and wherein the cover (127, 227) and the bottom wall (124) are partially spaced apart from each other along the direction of the axis (O) to form a side opening (162, 262) to form.,

Description

Technical FieldThe present invention relates to a gas sensor including a protector for protecting a gas sensor element exposed to a gas to be measured from adhesion of water.Prior ArtA conventionally known gas sensor includes a gas sensor element for measuring the concentration of a specific gas such as NO x( nitrogen oxide) or oxygen contained in an exhaust gas from an automobile or the like on the basis of the electromotive force generated depending on the differential concentration of the specific gas and varying in magnitude with the differential concentration. The gas sensor is mounted for use on, for example, an exhaust pipe of an automobile or the like. Because the gas sensor element becomes high in temperature by exposure to the hot exhaust gas and applying heat from a heater or the like, water contained in the exhaust gas or condensed water adhering to the inner surface of the exhaust pipe may adhere to the gas sensor element, so that the gas sensor element may crack or crack due to thermal shock.Therefore, a technique for protecting the gas sensor element from adhesion of water has been developed, in which the gas sensor element is covered with a protection member (see, for example, Patent Document 1). This protective member has a two-member structure and includes an inner protective member and an outer protective member. A gas to be measured is introduced into the protector through gas introduction holes formed in the inner and outer protector members. Further, gas discharge holes are formed in side walls of a recess part of a front end wall of the inner protector, thereby preventing a water droplet struck from the surrounding atmosphere (from the front side of the gas sensor element) from coming into direct contact with the gas sensor element. Further, according to this technique, a tapered part formed between a bottom wall and a side wall of the inner protector protrudes forward from the outer protector.Further relevant prior art is disclosed in the following documents: DE 10 2004 050 630 A1, DE 102 20 783 A1 and US 2008 0067066 A1.Documents from the Prior ArtPatent DocumentsPatent Document 1: Japanese Patent Application Laid-Open (kokai) No. JP 2008-96419 APatent Document 2: Laid-Open Application JP2010-523989 ASUMMARY OF THE INVENTIONProblem of the InventionHowever, in the gas sensor of Patent Document 1, since the bottom wall provided at the front end of the tapered part is formed to be relatively large as compared with the tapered part, generation of a negative pressure in the vicinity of the tapered part is less intensive. Therefore, difficulties are encountered in discharging the exhaust gas introduced into the inner space of the inner protector into the surrounding atmosphere through the gas discharge holes. This may impair the sensitivity of the gas sensor.In this connection, a gas sensor having a protective cap has been developed that includes a tubular sensor element chamber for covering a sensor element, a front chamber surrounding the outside of the sensor element chamber, and a wedge-like nozzle (a means for flux acceleration) at the front end of the sensor element chamber (see, for example, Patent Document 2). In this gas sensor, the nozzle has an outlet hole opening that opens at the front end and accelerates a flow in the sensor element chamber to discharge particulates and condensed liquid from inside the sensor element chamber through the outlet hole.However, in the gas sensor of Patent Document 2, since the outlet hole opening at the front end faces the sensor element, there is a problem that a water droplet flying from the surrounding atmosphere (from the front side of the gas sensor element) may come into contact with the sensor element through the outlet hole.The present invention aims to solve the above problem, and an object of the invention is to provide a gas sensor with a protector that has significantly improved gas execution performance to improve the sensitivity of the gas sensor, and that can effectively protect a gas sensor element from adhesion of water.Problem Solution of the InventionIn order to achieve the above object, a gas sensor according to the present invention includes a gas sensor element extending in a direction of an axis and having a detection part at its front end part for detecting a gas to be measured; a tubular metal shell that holds the gas sensor element such that the detection part can protrude from its front end; and a protection member that is fixed to a front end part of the metal shell and includes the detection part. The protective member includes an inner protective member and an outer protective member. The inner protector houses the detection part of the gas sensor element in its internal space, and includes a tubular side wall having an internal gas introduction hole for introducing the gas to be measured into the internal space, and a bottom wall provided at a front end of the side wall. The outer protector includes the inner protector with a clearance between the outer protector and the side wall of the inner protector, and includes a tubular side wall having an outer gas introduction hole disposed more forward than the inner gas introduction hole with respect to the direction of the axis and configured to introduce the gas to be measured into the clearance. The outer protector includes a frusto-conical tapered wall which is tapered forward from a front end of the side wall with respect to the direction of the axis, and an outer gas discharge hole which is formed inside a front end edge of the tapered wall and connects between the inside and the outside of the outer protector. When SLrepresents an area defined by the front end edge of the tapered wall, the outer gas discharge hole has an area S satisfying the relational expression 1 / 2× SL≤S≤S. The bottom wall of the inner protector has a bottom wall opening; the inner protector has a cover part covering the bottom wall opening from a rear side with respect to the direction of the axis and recessed behind the bottom wall with respect to the direction of the axis; and the cover part and the bottom wall are formed partially spaced apart from each other along the direction of the axis to form a side opening.In the gas sensor having this configuration, the outer protector has the truncated cone-like tapered wall that tapers forward from the front end of the side wall with respect to the axis direction, and the outer gas discharge hole in the truncated cone base opens at the front end of the outer protector so as to satisfy the relational expression 1 / 2× SL≤S≤S. Thus, a strong negative pressure is generated in the vicinity of the front end of the tapered wall on the downstream side with respect to a flow of the gas to be measured, thereby intensifying the effect of sucking out the gas to be measured introduced into the inner space of the inner protector from the outer gas discharge hole via the bottom wall opening of the inner protector. Thereby, the sensitivity of the gas sensor can be improved. In the case of the relational expression S<1 / 2<SL, a bottom wall at the truncated cone base of the front end of the outer protector is relatively large as compared with the tapered wall, which may result in the above effect not being obtained.When the outer gas discharge hole is formed in the truncated cone base at the front end of the outer protective member, it is difficult to provide a recessed portion as described in Patent Document 1.In the gas sensor of the present invention, the bottom wall opening is formed in the bottom wall of the inner protector so as to be covered by the cover part from the rear side with respect to the direction of the axis. Owing to this configuration, a water droplet flying along the direction of the axis into the bottom wall opening through the outer gas discharge hole is blocked by the cover part. Thus, it is unlikely that the water droplet or the like directly impinges on the gas sensor element, so that the gas sensor element can be protected from adhesion of water.Further, the gas sensor of the present invention may be configured such that the bottom wall opening and the outer gas discharge hole overlap each other as viewed from the direction of the axis.In a gas sensor configured in this manner, because the bottom wall opening (the cover part) and the outer gas discharge hole at least partially overlap each other, and because the bottom wall opening and the outer gas discharge hole are disposed in close proximity to each other, the gas to be measured can flow from the inner protector to the surrounding atmosphere along the direction of the axis. Thereby, the sensitivity of the gas sensor can be further improved.Further, the gas sensor of the present invention may be configured such that the side opening and the outer gas discharge hole overlap each other as viewed from the direction of the axis.In a gas sensor configured in this manner, since the side opening of the inner protector and the outer gas discharge hole at least partially overlap each other, the side opening and the outer gas discharge hole may be disposed in close proximity to each other. That is, compared to a configuration in which only the bottom wall opening and the outer gas discharge hole overlap each other, the gas to be measured can better flow from the inner protector to the surrounding atmosphere along the direction of the axis. Thereby, the sensitivity of the gas sensor can be further improved.Further, the gas sensor of the present invention may be configured such that the side wall of the inner protector has a drain hole, the drain hole not overlapping with the outer gas introduction hole in the position along the direction of the axis but overlapping with the side opening in the position along the direction of the axis.With such a configured gas sensor, even when a water droplet penetrates through the outer gas introduction hole of the outer protector, owing to the drainage hole in the side wall of the inner protector, the water droplet can be introduced into the inner protector through the drainage hole, thereby preventing the water droplet from entering through the inner gas introduction hole. Further, since the drain hole and the side opening are disposed in close proximity to each other, the water droplet that has entered the inner protector through the drain hole is immediately and smoothly discharged through the side opening (to the outer gas discharge hole). Because the outflow hole does not overlap with the outer gas introduction hole, a gaseous component that has passed through the outer gas introduction hole is prevented from directly passing through the outflow hole, so that the amount of the gas to be measured to be introduced into the inner protector through the inner gas introduction hole is maintained, thereby in turn maintaining the sensitivity of the gas sensor.Further, the gas sensor of the present invention may be configured such that a minimum width of the outer gas discharge hole is larger than a maximum width of the side opening along the direction of the axis.With such a configured gas sensor, the gas to be measured that has flowed out of the inner protector through the side opening can be discharged to the surrounding atmosphere without occurrence of stagnation in the outer protector that might otherwise occur due to throttling by the outer gas discharge hole. In this way, deterioration in sensitivity of the gas sensor can be suppressed.Further, the gas sensor of the present invention may be configured such that the minimum width of the outer gas discharge hole is 2 mm or more. With this configuration, a water droplet that has passed through the side opening can be easily discharged through the outer gas discharge hole without stagnation occurring in the outer protector. If the minimum width of the outer gas discharge hole is less than 2 mm, this effect may not be obtained.Further, the gas sensor of the present invention may be configured such that the maximum length of the side opening along the direction of the axis is 1.6 mm or more. Such a configuration enhances the effect of leading the gas to be measured introduced into the internal space to the outside of the inner protector (inside the outer protector), whereby the sensitivity of the gas sensor can be improved. If the maximum axial length of the side opening is less than 1.6 mm, this effect may not be obtained.Further, the gas sensor of the present invention may be configured such that the side wall of the inner protector includes a stepped part having a front facing surface and expanding radially outward along a rearward direction such that the stepped part is disposed within a range along the direction of the axis between the outer gas introduction hole and the inner gas introduction hole.In such a configured gas sensor, when the gas to be measured with water contained therein is introduced through the outer gas introduction hole, a gaseous component whose specific gravity is small rises and is introduced into the inner protector through the inner gas introduction hole while the water meets and drips the stepped part (the front-facing surface) of the inner protector. This can prevent water from entering the inner protector.Effect of the InventionThe present invention can considerably improve gas execution performance to improve sensitivity of a gas sensor, and can effectively protect a gas sensor element from adhesion of water.Brief Description of the DrawingsFIG. 1 is a partial sectional view along the axis direction of a gas sensor according to a first embodiment of the present invention. FIG. 2 is an enlarged partial sectional view of a protector. FIG. 3 is a perspective view of a cover part and a bottom wall opening in a bottom wall of an inner protective member. FIGS. 4A and 4B are enlarged partial sectional views of the front end edge of a tapered wall and the vicinity thereof. FIG. 5 shows the results of numerical simulation of the gas pressure distribution observed when the gas sensor according to the first embodiment of the present invention is placed in a gas flow. FIG. 6 shows the results of a numerical simulation of the gas pressure distribution observed when a gas sensor having a conventional protector is placed in a gas flow. FIG. 7 is a plan view showing the state of overlapping the bottom wall opening (cover part) of the inner protective member and an outer gas discharge hole of an outer protective member when viewed from the rear side of the gas sensor forward. FIG. 8 is a sectional view showing the positional relationship along the direction of the axis between the outflow holes, the inner gas introduction holes, the outer gas introduction holes, the side openings, and a stepped part of a side wall of the inner protector. FIG. 9 is an enlarged partial sectional view of a protector of a gas sensor according to a second embodiment of the present invention. FIG. 10 is a perspective view of the deck portions and bottom wall openings in a bottom wall of an inner protector in the second embodiment. FIG. 11 is a plan view showing a state of overlapping of the bottom wall openings (covers) of the inner protective member and an outer gas discharge hole of an outer protective member when viewed forward from the rear side of the gas sensor of the second embodiment.Embodiments of the InventionHereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a partial sectional view taken along an axis O direction of a gas sensor 1 according to a first embodiment of the present invention. The gas sensor 1 is a sensor for detecting the air / fuel ratio over a full range, which includes a gas sensor element 10 and is mounted on an exhaust pipe of an automobile (not shown). A detection part 11 of the gas sensor element 10 is exposed to the exhaust gas flowing through the exhaust pipe to detect the air / fuel ratio of the exhaust gas from the oxygen concentration in the exhaust gas.In the following description, the direction of the axis O of the gas sensor 1 is referred to as a vertical direction, the side toward the detection part 11 of the gas sensor element 10 of the gas sensor 1 is referred to as a front side of the gas sensor 1, and the side toward a rear end part 12 of the gas sensor element 10 is referred to as a rear side (near side) of the gas sensor 1.The gas sensor element 10 has a narrow plate-like shape extending in the direction of the axis O. The gas sensor element 10 has the shape of a substantially rectangular and columnar laminate in which the gas detection body for detecting the oxygen concentration and a heater body for rapidly activating the gas detection body by heat application are fixed to each other (the left-right direction of FIG. 1 corresponds to the thickness direction of the plate, while the front-rear direction of FIG. 1 corresponds to the width direction of the plate). The gas sensing body includes a solid electrolyte body mainly containing zirconia earth and electrodes mainly containing platinum (these members are not shown). The electrodes are disposed on the detection part 11 at a front end part of the gas sensor element 10. In order to protect the electrodes from damage by the exhaust gas, the detection part 11 of the gas sensor element 10 is covered with a protective layer 15. The rear end portion 12 of the gas sensor element 10 has five electrode pads 16 (one of which is shown in FIG. 1 ) to enable external connection with electrodes extending from the gas sensing body and the heater body. In the description of this embodiment, the gas sensor element 10 corresponds to the "gas sensor element" of the present invention. Strictly speaking, however, the heater is not a required component of the gas sensor element, so that the gas detection body corresponds to the "gas sensor element" of the present invention.A tubular metal cup 20 having a closed bottom is disposed at a position corresponding to a forward portion of a central trunk portion 13 of the gas sensor element 10 such that the gas sensor element 10 is inserted through the inside of the metal cup 20, and the detection portion 11 protrudes from an opening 25 formed in the bottom of the metal cup 20. The metal cup 20 is a member for holding the gas sensor element 10 in a metal shell 50, and a front end peripheral part 23 at a peripheral part of the bottom of the metal cup 20 is tapered to a tubular wall part of the metal cup 20. The talc ring 22 is compressed in the metal cup 20 to closely fill an associated space, thereby holding the gas sensor element 10 in position in the metal cup 20.An assembly of the metal cup 20 and the gas sensor element 10 is surrounded and held by the tubular metal shell 50. The metal shell 50 is configured to fixedly hold the gas sensor 1 to an exhaust pipe of an automobile (not shown). The metal shell 50 is formed of a low carbon steel such as SUS430, and has an external threaded part 51 provided on an outer circumferential surface of the metal shell 50 on the side facing the front end of the metal shell 50. The metal shell 50 has a front end engaging portion 56 which is disposed in front of the male threaded portion 51 and engages with the protective member 100, which will be described later. The metal shell 50 further includes a tool engaging portion 52 formed on an axially central portion of the outer circumferential surface of the metal shell 50 and engaged by a mounting tool. In order to prevent leakage of gas when the gas sensor 1 is attached to the exhaust pipe, a gasket 55 is fitted on a part of the metal shell 50 between the front end surface of the tool engaging part 52 and the rear end of the male threaded part 51. The metal shell 50 further includes a rear end engagement part 57 that is disposed behind the tool engagement part 52 and with which a below-described tubular jacket 30 engages, and a crimping part 53 that is disposed behind the rear end engagement part 57 and is configured to crimp-hold the gas sensor element 10 in the metal shell 50.The metal shell 50 has a stepped portion 54 on its inner circumferential surface at a position substantially corresponding to the male threaded portion 51. The front end peripheral part 23 of the metal cup 20 holding the gas sensor element 10 engages with the stepped part 54. Further, a talc ring 26 is placed in the metal shell 50 along the inner periphery of the metal shell 50 toward the rear end of the metal cup 20 such that the gas sensor element 10 is inserted through the talc ring 26. A tubular collar 27 is fitted into the metal shell 50 such that the talc ring 26 is pressed from its rear end. The cuff 27 has a step-shaped shoulder part 28 formed on the outer circumferential surface of a rear end part of the cuff 27. An annular crimp packing 29 is disposed on the shoulder portion 28. In this state, the crimping part 53 of the metal shell 50 is crimped so as to press the shoulder part 28 of the collar 27 forward via the crimp packing 29. Pressing the collar 27 crushes the talc ring 26 in the metal shell 50 to tightly fill an associated space. By the talc ring 26 and the talc ring 22 previously placed in the metal cup 20, the metal cup 20 and the gas sensor element 10 are held in position in the metal shell 50.The rear end part 12 of the gas sensor element 10 protrudes rearward beyond the rear end (the crimping part 53) of the metal shell 50. The rear end portion 12 is covered by a tubular separator 60 made of an electrically insulating ceramic. The separator 60 holds in its interior five connection terminals 61 (one of which is shown in FIG. 1 ) electrically connected to the five electrode pads 16 at the rear end part 12 of the gas sensor element 10. Further, the separator 60 protectively receives connections between the connection terminals 61 and five corresponding lead wires 65 (three of which are shown in FIG. 1 ) extending outward from the gas sensor 1.The tubular casing 30 is disposed so as to surround the rear end portion 12 of the gas sensor element 10 on which the separator 60 is fitted. The tubular jacket 30 is formed of stainless steel (e.g., SUS304). A front open part 31 of the tubular shell 30 engages with the outer periphery of the rear end engaging part 57 of the metal shell 50. The open end 31 is crimped radially inward, and laser welding is performed on the open end 31 along the entire outer periphery of the open end 31, thereby connecting the open end 31 to the rear end engagement part 57. In this way, the tubular shell 30 and the metal shell 50 are joined.A tubular metal holder 70 is disposed in the space between the tubular casing 30 and the separator 60. The metal bracket 70 has a holding part 71 formed by bending a rear end of the metal bracket 70 inward. The separator 60 is inserted through the metal holder 70 such that a flange part 62 formed on the outer periphery of the rear end part of the separator 60 engages with the holding part 71, so that the separator 60 is held by the holding part 71. In this state, a part of the tubular jacket 30 where the metal holder 70 is disposed is crimped inward, whereby the metal holder 70 holding the separator 60 is fixed to the tubular jacket 30.A rubber packing 75 made of a fluorine-containing rubber is fitted into an opening at the rear end of the tubular jacket 30. The rubber seal 75 has five insertion holes 76 (one of which is shown in FIG. 1 ). The five lead wires 65 extending outward from the separator 60 are air-tightly inserted through the corresponding insertion holes 76. While the rubber packing 75 pushes the separator 60 forward, a part of the tubular jacket 30 corresponding to the rubber packing 75 is crimped radially inward, thereby fixing the rubber packing 75 to the rear end of the tubular jacket 30.The detection part 11 of the gas sensor element 10 held by the metal shell 50 protrudes from a front end part (the front end engagement part 56) of the metal shell 50. The protector 100 is fitted on the front end engaging part 56 of the metal shell 50 to protect the detection part 11 of the gas sensor element 10 from contamination by deposits (harmful substances such as fuel ash and oil) in the exhaust gas and from breakage due to adhesion of water contained in the exhaust gas or condensed water and adhering to the inner surface of the exhaust pipe. The protector 100 is fixed to the front end engagement part 56 by spot welding or laser welding. Hereinafter, this protector 100 will be described with reference to FIGS. 2, 3 to 4.As shown in FIG. 2, the protective member 100 has a two-membered structure, and includes a tubular inner protective member 120 having a closed bottom and an outer protective member 110. The inner guard member 120 has a bottom wall 124 and a side wall 122. The outer protector 110 has a tubular side wall 112 radially surrounding the inner protector 120 with a clearance (hereinafter referred to as "gas separation chamber" 119) relative to the outer circumferential surface of the inner protector 120.The inner protector 120 has a smaller outer diameter than the front end engaging portion 56 of the metal shell 50, and includes an open end portion 121 which is an end portion on a side toward the opening (on a side toward a proximal end) and the diameter of which is expanded to engage with the outer periphery of the front end engaging portion 56. Laser welding is performed on the open end part 121 along the entire outer periphery of the open end part 121, thereby fixing the inner protector 120 to the front end engagement part 56 of the metal shell 50. Further, the side wall 122 of the inner protector 120 has a plurality of (six in the present embodiment) inner gas introduction holes 130 formed therein along the circumferential direction and formed toward the open end part 121 with respect to the direction of the axis O. The inner gas introduction holes 130 are provided to introduce mainly a gaseous component of the exhaust gas introduced into the gas separation chamber 119 through the gas introduction holes 115 of the below-described outer protector 110 into the inside of the inner protector 120, i.e., into a gas detection chamber 129 (corresponding to the "inner space" according to the claims) to which the detection part 11 of the gas sensor element 10 is exposed.Further, the side wall 122 of the inner protector 120 has a plurality of (four in the present embodiment) drain holes 150 formed therein along the circumferential direction and arranged toward the front end of the side wall 122. The outflow holes 150 are provided to discharge water (water droplets) and the like in the exhaust gas introduced into the gas separation chamber 119 to the outside of the inner gas shield member 120 via the gas detection chamber 129. The outflow holes 150 are disposed in front of the inner gas introduction holes 130.The bottom wall 124 of the inner protector 120 is substantially parallel to a plane perpendicular to the direction of the axis O. A (central) part of the bottom wall 124 is recessed rearward so as to be pushed axially inward and form a cover part 127. More specifically, as shown in FIG. 3, two parallel slits 127 aare formed in a central part of the bottom wall 124. A strip-shaped part of the bottom wall 124 between the slits 127a is pushed rearward to form the projecting cover part 127. The cover part 127 and the bottom wall 124 are spaced apart from each other along the direction of the axis O at the slits 127a, while the cover part 127 and the bottom wall 124 are connected to each other at short sides 127b (by line segments connecting the ends of the opposing slits 127a).In this way, the cover part 127 is held by the bottom wall 124 in a state in which a portion of the cover part 127 is separated from the bottom wall 124 along the direction of the axis O ("the cover part and the bottom wall are partially spaced apart from each other along the axis direction" according to the claims). A bottom wall opening 160 opens in a part of the bottom wall 124 corresponding to the cover part 127 as viewed in the direction of the axis O. Further, side openings 162 at an angle to the direction of the axis O are formed in respective regions (associated with the slits 127 a) in which the cover part 127 and the bottom wall 124 are spaced apart from each other along the direction of the axis O, and communicate with the bottom wall opening 160. The bottom wall opening 160 allows exhaust gas and water droplets introduced into the gas sensing chamber 129 to be discharged to the outside of the inner protector 120 through the side openings 162.When the maximum length of the side openings 162 along the direction of the axis O is 1.6 mm or more, the effect of executing exhaust gas introduced into the gas detection chamber 129 to the outside of the inner protector 120 (into the outer protector 110) is enhanced, whereby the sensitivity of the gas sensor 1 can be improved.This is based on the results of the following sensitivity evaluation test.In particular, sensitivity was tested using the following method. The theoretical air / fuel ratio (air / gasoline ratio) 14.7 was represented by a λ value of 1, forcing a transition between the rich condition (λ=0.97) and the lean condition (λ=1.03). The time until a change of the output value of the gas sensor 1 to a value corresponding to λ=1 after the change from the rich condition to the lean condition was measured. For this evaluation, the gas sensor 1 was mounted on the exhaust pipe of a 4-cylinder engine with a stroke of 2000 cm 3 and the engine was operated at a speed of 2000 U / min. The gas sensor 1 was mounted on the exhaust pipe at a position where the temperature of the engine exhaust gas was about 450°.As evaluation samples, gas sensors 1 having a maximum length of the side openings 162 along the direction of the axis O of 0.9 mm, 1.2 mm, and 1.6 mm, respectively, were used. The samples were evaluated by the above-mentioned evaluation method.The samples each gave a time period of 620 s (length along the direction of the axis O: 0.9 mm), a time period of 585 s (length along the direction of the axis O: 1.2 mm) and a time period of 560 s (length along the direction of the axis O: 1.6 mm). At the time of 560 sec, the gas sensor 1 provided sufficient sensitivity for actual use.Further, the outer protector 110 has a larger outer diameter than the front end engagement portion 56 of the metal shell 50, and includes an open end portion 111 which is an end portion on a side toward the opening (on a side toward a proximal end) and engages with the outer periphery of the front end engagement portion 56 (specifically, the outer periphery of the open end portion 121 of the inner protector 120). Laser welding is performed on the open end part 111 along the entire outer periphery of the open end part 111, whereby the outer protector 110 is fixed to the front end engagement part 56 of the metal shell 50. Further, the side wall 112 of the outer protective member 110 has a plurality of (six in the present embodiment) outer gas introduction holes 130 formed therein along the circumferential direction and arranged toward the front end with respect to the direction of the axis O. The outer gas introduction holes 130 are provided to introduce gas into the gas separation chamber 119 from the surrounding atmosphere. The outer gas introduction holes 115 are disposed in front of the inner gas introduction holes 130 with respect to the direction of the axis O.The outer protector 110 has a frusto-conical tapered wall 117 which tapers forwardly from the front end of the side wall 112 with respect to the direction of the axis O. In addition, the outer protector 110 has an outer gas discharge hole 170 formed inside a front end edge 117 sof the tapered wall 117 and connecting the inside and the outside of the outer protector 110.As shown in FIG. 4A, in the first embodiment, the front end edge 117 sof the tapered wall 117 and the periphery of the outer gas discharge hole 170 coincide with each other, that is, the outer gas discharge hole 170 is formed by cutting out the entire truncated cone base (an imaginary plane surrounded by the front end edge 117 s) of the tapered wall 117 and extends through the entire truncated cone base in the direction of the axis O. Thus, when SLrepresents the area defined by the front end edge 117 sof the tapered wall 117, the area S of the outer gas discharge hole 170 is equal to SL. However, according to the present invention, it is sufficient that the area S satisfies the relational expression 1 / 2× SL≤S≤S. For example, as shown in FIG. 4B, the following configuration may be adopted: the outer gas discharge hole 170 is disposed radially inward of the front end edge 117 sof the tapered wall 117, and an outer bottom wall 118 is formed to extend from the front end edge 117 sto the periphery of the outer gas discharge hole 170.Preferably, the minimum width of the outer gas discharge hole 170 is 2 mm or more (when the outer gas discharge hole 170 is circular, the area of the outer gas discharge hole 170 is 3.14 mm 2 or more) for the following reason: water droplets that have passed through the side openings 162 are likely to pass through the outer gas discharge hole 170 without stagnation in the outer protective member 110 due to their own weight, and are thereby discharged to the surrounding atmosphere.This is based on the results of the following evaluation test.Specifically, the evaluation samples were assemblies each configured such that the outer protective member 110 was fixed to the metal shell 50. The outer protecting members 110 of the samples had a minimum width of the outer gas discharge hole 170 of 0.1 mm, 0.15 mm, and 0.2 mm, respectively.The samples were evaluated using the following method. Water was dropped into the outer protective member 110 from the side toward the metal shell 50 to check whether the water is discharged through the outer gas discharge hole 170 of the outer protective member 110. In addition, the amount of water dropped was measured until water was discharged through the outer gas discharge hole 170 of the outer protective member 110.The amounts of the dropped water were 600 μl (minimum width: 0.1 mm), 600 μl (minimum width: 0.15 mm), and 280 μl (minimum width: 0.2 mm). Thus, it could be found that the water droplets are likely to be discharged to the surrounding atmosphere through the outer gas discharge hole 170 at a minimum width of the outer gas discharge hole 170 of the outer protective member 110 of 0.2 mm or more.Preferably, the taper angle of the tapered wall 117 is between 30° and 60° for the following reason: When such a taper angle is used, even if the angle at which the gas sensor 1 is mounted on an object body (such as an exhaust pipe) changes, the tapered wall 117 has an angle to the gas flow in the object body, so that a backward gas flow into the outer protector 110 through the outer gas discharge hole 170 is unlikely.And, when the minimum width of the outer gas discharge hole 170 is provided larger than the length of the side openings 162 along the direction of the axis O, the exhaust gas having passed from the gas detection chamber 12 through the side openings 162 can be discharged to the surrounding atmosphere without stagnation occurring in the outer protector 110, which might otherwise be due to throttling by the outer gas discharge hole 170. Thus, deterioration of the sensitivity of the gas sensor 1 can be avoided.That is, the outer protector 110 includes the frusto-conical tapered wall 117 that tapers forward from the front end of the side wall 112 with respect to the direction of the axis O, and the outer gas discharge hole 170 that opens in the frusto-conical base of the tapered wall 117 so as to satisfy the relational expression 1 / 2× SL≤S≤S. Thus, as described later, a strong negative pressure is generated in the vicinity of the downstream side with respect to a flow of the gas to be measured. The negative pressure effectively acts on the gas detection chamber 129 and enhances the effect of sucking out the exhaust gas introduced into the gas detection chamber 129.And, since the effect of sucking out the exhaust gas through the outer gas discharge hole 170 is intensified, even if the angle at which the gas sensor 1 is mounted on an object body (such as an exhaust pipe) changes, a backward gas flow through the outer gas discharge hole 170 into the outer protector 110 is unlikely. Thus, the gas to be measured can be detected stably without varying the sensitivity. In particular, when the gas sensor 1 is mounted at such a mounting angle that the front end of the gas sensor 1 is oriented upstream with respect to the gas flow while the rear end of the gas sensor 1 is oriented downstream, a backward flow of water and gas into the protector can be effectively suppressed.In the case of the relational expression 1 / 2× SL>S, the outer bottom wall 118 is formed relatively large as compared with the tapered wall 117, thereby mitigating the shape effect associated with the taper, and thus decreasing the negative pressure area in the vicinity of the front end of the tapered wall 117 on the downstream side with respect to the flow of the gas to be measured.FIGS. 5 and 6 show the results of numerical simulation of the gas pressure distributions that can be respectively observed when the gas sensor 1 according to the first embodiment of the present invention and a gas sensor having a conventional protector (as described in Patent Document 1) are placed in a gas flow. For the numerical simulation, turbulence analysis was performed using the Navier-Stokes equation and fluid analysis software (product name: STAR-CD) from CDadco JAPAN Co., LTD. (CAD). In FIGS. 5 and 6, a dark colored region P has a strong negative pressure (the black region indicated by the arrow P in the gradation in the legends of FIGS. 5 and 6 ).As shown in FIG. 5, in the outer protector 110 having, at its front end, the frusto-conical tapered wall 117 tapered forward with respect to the direction of the axis O and the outer gas discharge hole 170 opened in the frusto-conical base of the tapered wall 117 so as to satisfy the relational expression 1 / 2× SL≤S≤S (S=SL in FIG. 5 ), a strong negative pressure is generated in the vicinity of the front end of the tapered wall 117 on the downstream side with respect to the front end of the tapered wall 117 on the downstream side with respect to the gas flow (negative pressure range P).In contrast, in the case of the conventional protector of FIG. 6, the outer wall corresponding to the outer bottom wall 118 of FIG. 4B is formed relatively large as compared with the tapered wall corresponding to the tapered wall 117 of FIG. 4B (S=1 / 20 SL). Thus, it is understood from Fig. 6 that the negative pressure region P decreases in the vicinity of the leading end of the tapered wall on the downstream side with respect to the gas flow.Further, FIG. 7 shows a state of overlapping the bottom wall opening 160 (the cover part 127) of the inner protective member 120 and the outer gas discharge hole 170 of the outer protective member 110 forward as viewed from the rear side of the gas sensor 1.As shown in FIG. 7, the bottom wall opening 160 (the cover part 127) and the outer gas discharge hole 170 at least partially overlap each other (hatched area in FIG. 7 ).Further, as shown in FIG. 7, the side openings 162 and the outer gas discharge hole 170 partially overlap each other.Because the bottom wall opening 160 and the outer gas discharge hole 170 at least partially overlap each other and the bottom wall opening 160 and the outer gas discharge hole 170 are disposed in close proximity to each other, the gas to be measured can flow from the inner protector 120 to the surrounding atmosphere along the direction of the axis O. Thereby, the sensitivity of the gas sensor 1 can be further improved.Further, since the side openings 162 and the outer gas discharge hole 170 at least partially overlap each other and the side openings 162 and the outer gas discharge hole 170 are disposed in close proximity to each other, the gas to be measured can flow more easily from the inner protector 120 to the surrounding atmosphere along the direction of the axis O, compared to the configuration in which only the bottom wall opening 160 and the outer gas discharge hole 170 overlap each other. Thereby, the sensitivity of the gas sensor can be further improved.As mentioned above, the gas sensor 1 according to the first embodiment is configured as follows: 1) The outer protector 110 includes, at its front end, the frusto-conical tapered wall 117 that tapers forward with respect to the direction of the axis O, and the outer gas discharge hole 170 that opens in the frusto-conical base of the tapered wall 117 so as to satisfy the relational expression 1 / 2× SL≤S≤S. The bottom wall opening 160 is formed in the bottom wall 14 of the inner protector 120 so as to be covered from the rear side by the cover part 127.Thanks to the configuration mentioned above under 1), a strong negative pressure is generated in the vicinity of the front end of the tapered wall 117 on the downstream side with respect to the flow of the gas to be measured, thereby enhancing the effect of sucking out the exhaust gas introduced into the gas detection chamber 129 from the outer gas discharge hole 170 via the bottom wall opening 160. Thereby, the sensitivity of the gas sensor 1 can be improved.In addition, in the configuration mentioned under 2), the bottom wall opening 160 is formed in the bottom wall 14 of the inner protector 120 accommodated in the outer protector 110 so as to be covered by the cover part 12 from the rear side with respect to the direction of the axis O. With this configuration, when a water droplet or the like flies into the bottom wall opening 160 along the direction of the axis O through the outer gas discharge hole 170, the cover part 127 blocks the water droplet or the like. Therefore, the water droplet or the like is unlikely to directly reach the gas sensor element 10, so that the gas sensor element 10 can be protected from adhesion of water.As shown in FIG. 8, the outflow holes 150 of the inner protector 120 preferably do not overlap with the outer gas introduction holes 115 along the direction of the axis O, but the outflow holes 150 overlap with the side openings 162 along the direction of the axis O. Here, when it is said that "the outflow holes 150 of the inner protector 120 do not overlap with the outer gas introduction holes 115 in the position along the direction of the axis O", it means that the front ends of the outer gas introduction holes 115 are located behind the rear ends L 1 of the outflow holes 150. Here, when it is said that "the drain holes 150 overlap with the side openings 162 in the position along the direction of the axis O", it means that at least a part of each of the side openings 162 is located between the positions of the rear ends L 1 and the front ends L 2 of the drain holes 150.In the above-described configuration, because the drain holes 150 and the side openings 162 are disposed in proximity to each other, water W entering the inner protector 120 through the drain holes 162 is immediately and easily discharged through the side openings 162 (to the outer gas discharge hole 170). Further, since the outflow holes 150 do not overlap with the outer gas introduction holes 115, the gas to be measured having passed through the outer gas introduction holes 115 can be prevented from directly passing through the outflow holes 150, so that the amount of the gas to be measured to be introduced into the gas detection chamber 129 of the inner protector 120 through the inner gas introduction holes 130 can be maintained, whereby the sensitivity of the gas sensor 1 can be maintained.Further, as shown in FIG. 8, the side wall 122 of the inner protector 120 includes a stepped part 122D having a front facing surface and expanding radially outward along a rearward direction, the stepped part 122D being disposed within a range R along the direction of the axis O between the outer gas introduction holes 115 and the inner gas introduction holes 130. The region R extends between the front ends of the inner gas introduction holes 130 and the front ends of the outer gas introduction holes 115.In the above configuration, when the gas to be measured with water contained therein is introduced through the outer gas introduction holes 115, a gaseous component G whose specific gravity is low increases and is introduced into the inner protector 120 through the inner gas introduction holes 130, while the water W impinges and falls on the stepped part 122D (the front-facing surface) of the inner protector 120. Thereby, entry of water into the gas detection chamber 129 of the inner protector 120 can be prevented.Regarding the above drainage effect, when a front end corner 120 c(a transition between the side wall 122 and the bottom wall 124) is in contact with the inner surface of the tapered wall 117 of the outer protector 110, water collected between the inner protector 120 and the outer protector 110 (hatched area in FIG. 8 ) is difficult to drain (to the outer gas discharge hole 170).In order to improve the drainage effect, a clearance is preferably formed between the front end corner 120 cand the inner surface of the tapered wall 117.When a clearance is formed between the front end corner 120 cand the inner surface of the tapered wall 117, the gas to be measured introduced through the outer gas introduction holes 115 passes through the clearance. An intermediate space is thus not advantageous with respect to the sensitivity of the gas sensor 1. Thus, for the sensitivity of the gas sensor 1, the front end corner 120 cof the inner protector 120 is preferably in contact with the inner surface of the tapered wall 117 of the outer protector 110.Next, a gas sensor 2 according to a second embodiment of the present invention will be described with reference to FIGS. 9, 10 to 11. Because the gas sensor 2 according to the second embodiment is similar to the first embodiment except for the configuration of the cover parts 227 and the bottom wall openings 260 in the bottom wall 124 of the inner protector 120, configuration features similar to those of the first embodiment are denoted by the same reference numerals, and repeated description of these configuration features is omitted here.In FIG. 9, the bottom wall 124 of the inner guard member 120 is substantially parallel to the plane perpendicular to the direction of the axis O. Two substantially semicircular parts of the bottom wall 124 are recessed rearward so as to be pushed axially inward and form the corresponding cover parts 227. More specifically, as shown in FIG. 10, two parallel slits 227a are formed in the bottom wall 124. Two semicircular portions of the bottom wall 124 which are disposed between the slits 227a and face each other are pushed rearward to form the two deck portions 227. Each of the two cover members 227 opens radially outward on the side toward the slit 227a (the side opening 262) and takes the form of a hemisphere having a low profile. Each of the deck parts 227 and the bottom wall 124 are spaced apart from each other in the direction of the axis O at the corresponding slit 227a, and are connected to each other at the semicircular edge of the deck part 227. The two covers 227 are arranged such that their semicircular edges are in contact with each other at the center of the bottom wall 124, and their side openings 262 are oriented in radially opposite directions.Each of the cover parts 227 is held by the bottom wall 124 in such a state that a portion of the cover part 227 is spaced apart from the bottom wall 124 in the direction of the axis O (in a state in which "the cover part and the bottom wall are partially spaced apart from each other along the direction of the axis O", according to the claims). Further, side openings 262 are formed at an angle to the direction of the axis O in respective regions (associated with the slits 227 a) in which the cover parts 227 and the bottom wall 124 are spaced apart from each other along the direction of the axis O, and are connected to the respective bottom wall openings 260. The bottom wall openings 260 allow exhaust gas and water droplets introduced into the gas sensing chamber 129 to be discharged to the outside of the inner protector 120 through the side openings 262.In the second embodiment, because a plurality of (two) bottom wall openings 260 are provided, the total opening area of the bottom wall openings 260 is large, whereby the discharge of exhaust gas and water from the inner protector 120 can be further improved.FIG. 11 shows a state of overlapping the bottom wall openings 260 (the cover parts 227) of the inner protector 120 with the outer gas discharge hole 170 of the outer protector 110 as viewed forward from the rear side of the gas sensor 2.As shown in FIG. 11, the bottom wall openings 260 and the outer gas discharge hole 170 at least partially overlap each other (hatched area in FIG. 11 ). Because the bottom wall openings 260 and the outer gas discharge hole 170 are disposed in close proximity to each other with respect to a direction perpendicular to the direction of the axis O, the gas to be measured can flow from the inner protective member 120 to the surrounding atmosphere along the direction of the axis O. Thereby, the sensitivity of the gas sensor 2 can be further improved.Also in the second embodiment, a strong negative pressure is generated in the vicinity of the front end of the tapered wall 117 on the downstream side with respect to the flow of the gas to be measured, thereby intensifying the effect of sucking out exhaust gas and water droplets introduced into the gas detection chamber 129 from the outer gas discharge hole 170 through the bottom wall openings 260. Thereby, the sensitivity of the gas sensor 2 is improved. And, even when a water droplet or the like flies into the bottom wall openings 260 through the outer gas discharge hole 170 along the direction of the axis O, the covers 227 block the water droplet or the like. Therefore, the water droplet or the like is unlikely to directly hit the gas sensor element 10, so that the gas sensor element 10 can be protected from adhesion of water.The present invention is not limited to the above-described embodiments, which can be modified in various ways. For example, the bottom wall opening and the cover part of the inner protector are not limited to the above shapes and amounts. Thus, the cover portion may also be formed as follows: a slit similar to a simple or rectangular letter "U" is formed in the bottom wall of the inner protector, and the portion of the bottom wall surrounded by the slit is pushed rearward. In this case, a single portion of the cover member extends in the axis direction and is connected to the bottom wall.In the above-described embodiments, the outflow hole and the inner gas introduction hole are provided in the same outer protector side wall generatrix, and also the outer gas introduction hole is formed on an outer protector generatrix oriented in the same radial direction as the inner protector generatrix. The holes can, however, also be formed on different genetics. Or, a particular hole may be formed on another generatrix. Further, the number of the outer gas introduction holes and the number of the inner gas introduction holes are not limited to six, and the number of the outflow holes is not limited to four. Further, the size and shape of the holes can be determined arbitrarily.Further, the present invention can be similarly applied to oxygen sensors, NO x- sensors, HC sensors, temperature sensors, etc.List of Reference Numerals1, 2 Gas sensor 10 Gas sensor element 11 Detection part 50 Metal shell 100, 200 Protection member 110 Outer protection member 112 Side wall of outer protection member 115 Outer gas introduction hole 117 Tapered wall 117 sFront end edge of tapered wall 118 Outer bottom wall 119 Space 120 Inner protection member 122 Side wall of inner protection member 122D Stepped part of inner protection member 124 Bottom wall of inner protection member 127, 227 Cover part 129 Gas detection chamber (inner space of inner protection member) 130 Inner gas introduction hole 150 Drain hole 160, 260 Bottom wall opening 162, 262 Side opening 170 Outer gas discharge hole O Axis SL area defined by the front end edge of tapered wall S Opening area of outer gas discharge hole R Area

Claims

A gas sensor (1, 2) comprising: a gas sensor element (10) extending in a direction of an axis (O) and having, at its front end part, a detection part (11) for detecting a gas to be measured; a tubular metal shell (50) holding the gas sensor element (10) such that the detection part (11) can protrude from its front end; and a protection member (100, 200) fixed to a front end part of the metal shell (50) and including the detection part (11), wherein the protection member (100, 200) comprises: an inner protection member (120), A sensing part (11) of the gas sensor element (10) is accommodated in the internal space (129) thereof, and includes a tubular side wall (122) having an internal gas introduction hole (130) for introducing the gas to be measured into the internal space (129) and a bottom wall (124) at a front end of the side wall (122), and an outer protector (110) that accommodates the inner protector (120) with a clearance (119) between the outer protector (110) and the side wall (122) of the inner protector (120), and that includes a tubular side wall (112) having an outer gas introduction hole (115) provided in front of the inner gas introduction hole (130) with respect to the direction of the axis (O) and configured to introduce the gas to be measured into the clearance (119), wherein the outer protector (110) has a frustoconical shape, A tapered wall (117) which is tapered forward from a front end of the side wall (112) with respect to the direction of the axis (O), and comprises an outer gas discharge hole (170) which is formed inside a front end edge (117s) of the tapered wall (117) and establishes communication between the inside and the outside of the outer protector (110), wherein when SL represents an area defined by the front end edge (117s) of the tapered wall (117), the outer gas discharge hole (170) has an area (S) satisfying the relational expression 1 / 2 × SL ≤ S ≤ SL, and wherein the bottom wall (124) of the inner protector (120) has a bottom wall opening (160, 260), wherein the inner protector (120) has a cover part (127, 227), covering the bottom wall opening (160, 260) from a rear side with respect to the direction of the axis (O) and recessed behind the bottom wall (124) with respect to the direction of the axis (O), and wherein the cover part (127, 227) and the bottom wall (124) are partially spaced apart from each other along the direction of the axis (O) to form a side opening (162, 262).The gas sensor (1, 2) according to claim 1, wherein the bottom wall opening (160, 260) and the outer gas discharge hole (170) overlap each other as viewed from the direction of the axis (O).The gas sensor (1, 2) according to claim 1 or 2, wherein the side opening (162, 262) and the outer gas discharge hole (170) overlap each other as viewed from the direction of the axis (O).The gas sensor (1, 2) according to any one of claims 1 to 3, wherein the side wall (122) of the inner protector (120) has a drain hole (150), and the drain hole (150) does not overlap with the outer gas introduction hole (115) in the position along the direction of the axis (O) and overlaps with the side opening (162, 262) in the position along the direction of the axis (O).The gas sensor (1, 2) according to any one of claims 1 to 4, wherein the outer gas discharge hole (170) and the side opening (162, 262) are in such a dimensional relationship that a minimum width of the outer gas discharge hole (170) is larger than a maximum length of the side opening (162, 262) along the direction of the axis (O).The gas sensor (1, 2) according to any one of claims 1 to 5, wherein the minimum width of the outer gas discharge hole (170) is 2 mm or more.The gas sensor (1, 2) according to any one of claims 1 to 6, wherein the maximum length of the side opening (162, 262) along the direction of the axis (O) is 1.6 mm or more.The gas sensor (1, 2) according to any one of claims 1 to 7, wherein the side wall (122) of the inner protector (120) has a stepped part (122D) having a front facing surface and expanding radially outward along a rearward direction, the stepped part (122D) being disposed within a range (R) along the direction of the axis (O) between the outer gas introduction hole (115) and the inner gas introduction hole (130).

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