Gas sensor

Abstract

To provide a gas sensor capable of surely improving water resistance using a single-layer protector.SOLUTION: A gas sensor 1 comprises a sensor element 21 and a single-layer protector 51. The protector includes a large diameter part 51x having a tip-facing surface 51f1, a small diameter part 51y protruding from the large diameter part toward a tip side and connected to a tip-facing surface, and a plurality of gas introduction holes 56 formed on the tip-facing surface. A louver 51L extending toward the rear end side is provided radially inside the gas introduction hole. When a specific section along an axis O direction through the center of gravity G of the protector and equally dividing one louver circumferentially is viewed, to an intersection P of a virtual line V passing through the tip of the sensor element and perpendicular to the axial direction and an inner surface of a first peripheral wall, a line segment connecting a tip Q of a surface radially outside the louver and the intersection P is defined as L, and an angle formed between the line segment L and the tip-facing surface is defined as virtual installation angle α. In this case, actual attachment angle F of the louver is less than α.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a gas sensor having a protector that protects a detection element exposed to a gas to be detected from being wet. [Background technology]

[0002] Conventionally, gas sensors have been known in which a sensor element is held in a cylindrical metal shell and the tip end of the sensor element, which is exposed to the measured gas such as exhaust gas, is protected by a single or double protector. This protector has a gas inlet hole, but is required to be water-resistant to prevent condensed water mixed in the exhaust gas from reaching the sensor element and to have responsiveness to quickly introduce exhaust gas into the detection section of the sensor element. Here, the sensor element is heated by its own heater or by high-temperature exhaust gas, and if condensed water comes into contact with the sensor element, thermal shock may occur, which may cause the element to crack.

[0003] Therefore, a technology has been developed in which the protector is made single-layered to improve responsiveness, and a louver-shaped opening for introducing gas is provided in a horizontal step portion of the protector (Patent Document 1). According to this technology, no openings are provided in the cylindrical surface (circumferential wall) of the protector, so there are no openings along the direction of the flow of the gas to be detected. As a result, the gas to be measured changes its flow direction along the louvers inside the protector before reaching the detection element, and condensed water (water droplets) in the gas to be detected are trapped near the openings, preventing the condensed water from adhering to the detection element. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Laid-Open No. 2013-257192 Summary of the Invention [Problem to be solved by the invention]

[0005] As described above, to address the problem of water resistance that occurs with single-layer protectors, it is effective to make the opening for introducing gas in a louver shape, but it has been found that water resistance decreases depending on the louver angle. The present invention has been made in view of the above circumstances, and has as its object to provide a gas sensor that can reliably improve water resistance by using a single protector. [Means for solving the problem]

[0006] In order to solve the above problems, the gas sensor of the present invention comprises a sensor element extending in an axial direction and having a detection portion formed on a tip end side thereof for detecting a gas to be detected, a cylindrical metal shell surrounding and holding the sensor element in a radial direction, and a single-layer protector fixed to the tip end side of the metal shell, wherein the protector comprises a large diameter portion having a cylindrical first circumferential wall surrounding the tip end side of the sensor element and a tip-facing surface connected to the tip end side, a small diameter portion protruding from the large diameter portion to the tip end side in the axial direction and having a cylindrical second circumferential wall connected to the tip-facing surface and a bottom surface connected to the tip end side, and a plurality of gas sensors formed at intervals in the circumferential direction on the tip-facing surface. a gas introduction hole, and an outer diameter of the first peripheral wall is larger than an outer diameter of the second peripheral wall, and louvers extending toward a rear end side are provided radially inside the gas introduction hole, and when a specific cross section that passes through the center of gravity of the protector along the axial direction and divides one louver equally in the circumferential direction is viewed, an intersection P between an imaginary line that passes through the tip of the sensor element and is perpendicular to the axial direction and the inner surface of the first peripheral wall is defined as an intersection P, where a line segment connecting a tip Q of the radially outer surface of the louver and the intersection P is defined as L, and an angle between the line segment L and the tip-facing surface is defined as a virtual installation angle α, an actual installation angle F of the louvers is less than α.

[0007] If the louver is installed at a virtual installation angle α or more, the gas to be detected that flows from the gas inlet hole along the radially outer surface (inner wall) of the louver will hit the intersection P on the inner surface of the first peripheral wall, bounce off horizontally, and hit the sensor element, which may cause condensed water in the gas to be detected to adhere to the sensor element. Therefore, if F<α, even if the condensed water in the gas to be detected hits the intersection P and bounces off horizontally, it will only move toward the tip side of the sensor element, so that it is possible to reliably prevent the condensed water in the gas to be detected from adhering to the sensor element. It is difficult to imagine that the condensed water that hits the intersection point P reaches above the horizontal direction (toward the rear end) due to gravity.

[0008] In the gas sensor of the present invention, a three-dimensional model of the sensor element of the gas sensor, the metallic shell, and the protector with variously changed mounting angles of the louvers is constructed, and the gas sensor is attached to an exhaust pipe and air at 25°C is allowed to flow through it at a flow rate of 5 m / s. The state of gas replacement inside the protector is determined using a turbulence model of a computational fluid dynamics simulation. Here, a passive scalar substance with a defined O2 diffusion coefficient is allowed to flow in from the exhaust pipe inlet at time 0 seconds, and a steady-state single-phase flow analysis is performed to extract the replacement rate of the passive scalar substance at the tip of the sensor element over time. When the gas replacement time T when the mounting angle of the louvers is 70 degrees is T × 1.2, and the second virtual mounting angle of the louvers is β (where β < α), the mounting angle F of the louvers may be greater than or equal to β and less than α.

[0009] As a result of the above-mentioned fluid analysis, it was found that as the mounting angle F becomes smaller, the responsiveness tends to decrease. Therefore, under the condition F<α, which ensures water resistance as described above, if β≦F is also satisfied, both water resistance and responsiveness can be improved.

[0010] In the gas sensor of the present invention, the mounting angle F of the louver may be less than 58 degrees. In the gas sensor of the present invention, the mounting angle F of the louver may be set to be equal to or greater than 47 degrees and less than 58 degrees. [Effects of the Invention]

[0011] According to the present invention, a gas sensor can be obtained that can reliably improve water resistance by using a single protector. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a cross-sectional view of a gas sensor according to an embodiment of the present invention. [Figure 2] FIG. 2 is a partially enlarged cross-sectional view of FIG. [Figure 3] FIG. 1 is a plan view of the protector (gas introduction hole) viewed from the front end side to the rear end side. [Figure 4] FIG. 10 is a diagram showing the relationship between the installation angle of the louver and responsiveness, based on fluid analysis. DETAILED DESCRIPTION OF THE INVENTION

[0013] An embodiment of the present invention will be described in detail with reference to Fig. 1 to Fig. 3. Fig. 1 is a cross-sectional view of a gas sensor 1 according to the embodiment of the present invention, Fig. 2 is a partially enlarged cross-sectional view of Fig. 1, and Fig. 3 is a plan view of a protector 51 (gas introduction hole 56) viewed from the front end side to the rear end side.

[0014] In FIG. 1, the gas sensor (wide-range air-fuel ratio gas sensor) 1 includes a sensor element 21, a holder (ceramic holder) 30 having a through hole 32 penetrating in the axial direction O and through which the sensor element 21 is inserted, a metal shell 11 surrounding the ceramic holder 30 radially, and a protector 51. The tip side of the sensor element 21, where the detection portion 22 is formed, protrudes further toward the tip than the ceramic holder 30 and the metal shell 11. The sensor element 21 thus passed through the through hole 32 is fixed inside the metal shell 11 while maintaining airtightness in the front-to-rear direction by compressing a filler material (talc in this example) 41 arranged on the rear end face side (upper side in the figure) of the ceramic holder 30 in the front-to-rear direction via a sleeve 43 and a ring washer 45 made of an insulating material.

[0015] The rear end side 29 of the sensor element 21 protrudes rearward beyond the sleeve 43 and the metallic shell 11, and the electrode terminals 24 formed on the rear end side 29 are electrically connected by pressure contact with the terminal fittings 75 provided at the ends of the lead wires 71 that are drawn to the outside through a sealing material 85. The rear end side 29 of the sensor element 21, including the electrode terminals 24, is covered with an outer cylinder 81. This will be explained in more detail below.

[0016] The sensor element 21 extends in the direction of the axis O and is in the form of a strip (plate) with a detection section 22, consisting of detection electrodes (not shown) and adapted to detect a specific gas component in a gas to be detected, at its tip (lower side in the figure) facing the object to be measured. The cross section of the sensor element 21 is rectangular (rectangular) of a fixed size at both the front and rear ends, and is formed as an elongated structure made primarily of ceramic (solid electrolyte, etc.). The sensor element 21 itself is the same as a conventionally known sensor element, with a pair of detection electrodes forming the detection section 22 disposed at the tip end of the solid electrolyte (member), and an electrode terminal 24 connected to the detection electrodes and exposed at the rear end for connection to a lead wire 71 for extracting detection output.

[0017] In this example, a heater (not shown) is provided inside the front end of the ceramic material formed in a laminated state on the solid electrolyte (member) of the sensor element 21, and electrode terminals 24 for connecting lead wires 71 for applying voltage to the heater are formed and exposed on the rear end. Although not shown, these electrode terminals 24 are formed in a vertically elongated rectangular shape, and for example, at the rear end side 29 of the sensor element 21, three or two electrode terminals are lined up horizontally on the wide surfaces (both sides) of the strip plate. The detection portion 22 of the sensor element 21 is covered with a porous protective layer 23 made of alumina, spinel, or the like.

[0018] In addition, on both short sides of the sensor element 21, there are provided element introduction holes that communicate with the detection section 22 and introduce the gas to be detected into the detection section 22 (measurement chamber), and a porous diffusion resistance layer is arranged in the element introduction holes.

[0019] The metal shell 11 has a cylindrical shape with concentric front and rear ends but different diameters, and has a small-diameter cylindrical annular portion (hereinafter also referred to as the cylindrical portion) 12 at the front end for fitting and fixing a protector 51 (described later) thereto, and a larger-diameter screw 13 for fixing to an engine exhaust pipe is provided on the outer peripheral surface at the rear (upper part in the figure).Furthermore, behind this, there is a polygonal tool engaging portion 14 for screwing the sensor 1 with this screw 13.Furthermore, behind this tool engaging portion 14, there is a cylindrical portion 15 connected to which a protective tube (outer tube) 81 that covers the rear of the gas sensor 1 is fitted and welded, and behind this, there is a thin-walled cylindrical portion 16 for crimping that has a smaller outer diameter than the cylindrical portion 12.

[0020] 1, the crimping cylindrical portion 16 is bent inward for the purpose of post-crimping. A gasket 19 is attached to the underside of the tool engaging portion 14 for sealing when screwed in. On the other hand, the metallic shell 11 has an inner hole 18 penetrating therethrough in the direction of the axis O. The inner peripheral surface of the inner hole 18 has a tapered step portion 17 that tapers radially inward from the rear end side to the front end side.

[0021] A ceramic holder 30 made of insulating ceramic (e.g., alumina) and formed in a roughly short cylindrical shape is disposed inside the metal shell 11. The ceramic holder 30 has a front-facing surface 30a formed in a tapered shape that narrows toward the front end. A portion of the front-facing surface 30a near the outer periphery is engaged with the step portion 17, and the ceramic holder 30 is pressed from the rear end side by a sealing material 41, thereby positioning the ceramic holder 30 within the metal shell 11 and providing a clearance fit. On the other hand, the through hole 32 is provided in the center of the ceramic holder 30 and is a rectangular opening having substantially the same dimensions as the cross section of the sensor element 21 so that the sensor element 21 can pass through with almost no gap.

[0022] The sensor element 21 is inserted into the through-hole 32 of the ceramic holder 30 , and the tip 21 a of the sensor element 21 protrudes forward beyond the ceramic holder 30 and the tip 12 a of the metallic shell 11 . On the other hand, the tip of the sensor element 21 is covered with a single-layered, bottomed, cylindrical protector (protective cover) 51. The rear end of the protector 51 is fitted onto the cylindrical portion 12 of the metal shell 11 and welded thereto.

[0023] The protector 51 has a cylindrical first peripheral wall 51s that surrounds the tip side (detection unit 22) of the sensor element 21, a large diameter portion 51x that has a tip-facing surface 51f1 connected to the tip side, and a small diameter portion 51y that protrudes from the large diameter portion 51x to the tip side in the direction of the axis O. The small diameter portion 51y has a cylindrical second peripheral wall 51t that is connected to the tip-facing surface 51f1 and a bottom surface 51f2 that is connected to the tip side. The outer diameter of the first peripheral wall 51s is larger than the outer diameter of the second peripheral wall 51t. Furthermore, a plurality of gas introduction holes 56 are formed at equal intervals in the circumferential direction on the tip-facing surface 51f1 (FIG. 3).

[0024] Meanwhile, one gas discharge hole 53 is disposed on bottom surface 51f2. In this example, gas discharge hole 53 is disposed at the center of gravity of bottom surface 51f2, and this center of gravity can be calculated as the geometric center of a plane figure formed by projecting bottom surface 51f2 onto a plane. The gas exhaust hole 53 is positioned closer to the tip than the gas inlet hole 56, and the gas inside the protector 51 is sucked out through the gas exhaust hole 53 by the flow of the gas to be detected that flows through the mounting object (exhaust pipe, etc.) on which the gas sensor 1 is mounted, and the negative pressure causes the gas to be detected to be introduced into the protector 51 through the gas inlet hole 56.

[0025] The tip-facing surface 51f1 has louvers 51L cut and raised from a single cut toward the rear end at the position of each gas introduction hole 56, and the radially outer surface (inner wall) 51i (Figure 1) of the louvers 51L forms the gas introduction hole 56. Also, as shown in Figure 2, the tip Q of the inner wall 51i of the louver 51L is located radially inward from the rear end R, and the louver 51L is positioned radially inward from the gas introduction hole 56 so as to surround the gas introduction hole 56.

[0026] In this example, the louvers 51L are open only toward the inner surface of the first peripheral wall 51s (that is, only toward the outside in the radial direction). Here, "the louver 51L is open only toward the inner surface of the first peripheral wall 51s" means that, as shown in Figure 3, the gas introduction hole 56 is not visible when viewed radially outward from the center of gravity G of the tip-facing surface 51f1 (in other words, the louver 51L is always located on the line connecting the center of gravity G and the gas introduction hole 56). In this way, the detection gas introduced into the protector 51 from the gas inlet hole 56 is obstructed by the louver 51L, which prevents the detection gas (and the condensed water contained therein) from directly hitting the sensor element 21 inside the protector 51, thereby improving water resistance.

[0027] The center of bottom surface 51f2 is cut and raised toward the rear end along two parallel slits to form cover 53c, and gas discharge hole 53 is formed in the radial direction in the gap between bottom surface 51f2 and cover 53c. In this case, when protector 51 is viewed from the tip end side in the direction of axis O, gas discharge hole 53 is not directly visible, so that water droplets such as condensed water can be prevented from entering protector 51 through gas discharge hole 53.

[0028] 1, terminal fittings 75 provided at the tips of lead wires 71, which are drawn to the outside through a sealing material 85, are pressed against and electrically connected to the electrode terminals 24 formed on the rear end side 29 of the sensor element 21 by their spring properties. In the gas sensor 1 of this embodiment, the terminal fittings 75 including the pressure-contact portions are disposed opposite each other in respective housing portions provided in an insulating separator 91 disposed within the outer cylinder 81.

[0029] The separator 91 is restricted from moving radially and toward the tip end via a holding member 82 that is fixed by crimping inside the outer cylinder 81. The tip end of the outer cylinder 81 is fitted onto and welded to the cylindrical portion 15 on the rear end side of the metallic shell 11, thereby covering the rear of the gas sensor 1 in an airtight manner. The lead wire 71 is passed through a sealing material (e.g., rubber) 85 arranged inside the rear end of the outer tube 81 and pulled out to the outside, and the small-diameter cylindrical portion 83 of the outer tube 81 is crimped to reduce the diameter and compress this sealing material 85, thereby maintaining airtightness in this area.

[0030] Incidentally, a step 81d having a larger diameter at the front end is formed slightly rearward from the center of the outer cylinder 81 in the direction of the axis O, and the inner surface of this step 81d supports and pushes the rear end of the separator 91 forward. Meanwhile, a flange 93 formed on the outer periphery of the separator 91 is supported on a holding member 82 fixed to the inside of the outer cylinder 81, and the separator 91 is held in the direction of the axis O by the step 81d and the holding member 82.

[0031] Next, the characteristic features of the present invention will be described. As shown in Figure 2, L is the line segment connecting the tip Q of the radially outer surface (inner wall) 51i of the louver 51L to the intersection P between an imaginary line V that passes through the tip of the sensor element 21 and is perpendicular to the axis O direction and the inner surface of the first peripheral wall 51s. When the angle formed by the line segment L and the tip-facing surface 51f1 is defined as a virtual installation angle α (degrees), the actual installation angle F (degrees) of the louver 51L is set to be equal to or less than α.

[0032] If the louver 51L is installed at a virtual installation angle α or more, the gas to be detected that flows from the gas inlet hole 56 along the inner wall 51i of the louver 51L may hit the intersection P on the inner surface of the first peripheral wall 51s, bounce off horizontally, and hit the sensor element 21, which may cause condensed water in the gas to be detected to adhere to the sensor element 21. Therefore, if F<α is set, even if the condensed water hits the intersection P and bounces off horizontally, it will only move further towards the tip side than the sensor element 21, so that adhesion of the condensed water to the sensor element 21 can be reliably prevented. It is difficult to imagine that the condensed water that hits the intersection point P reaches above the horizontal direction (toward the rear end) due to gravity.

[0033] Here, the cross section in Figure 2 for determining the mounting angles α and F is a specific cross section that passes through the center of gravity of the protector 51 (= center of gravity G of the tip-facing surface 51f1) along the axis O direction and divides one louver 51L equally in the circumferential direction. This is because it is difficult to accurately determine the attachment angles α and F when a portion of louver 51L is cut or when the cross section is cut obliquely with respect to the direction in which louver 51L extends.

[0034] The reason why the tip Q is used as the reference when calculating the virtual mounting angle α is because the tip Q is the starting point of the louver 51L. The tip Q is also defined as a point on an extension line S of the outer surface of the tip-facing surface 51f1. The imaginary mounting angle α is the angle formed by the line segment L and the extension line S of the tip-facing surface 51f1. When the tip of the sensor element 21 is covered with the protective layer 23, the imaginary line V "passes through the tip of the sensor element 21" means that the imaginary line V passes through the tip of the main body of the sensor element 21 excluding the protective layer 23. This is because the main body of the sensor element 21 would be damaged by water.

[0035] The attachment angle F is the angle between the line segment LT and an extension line S, where LT is the line segment connecting the leading end Q and the rear end R of the inner wall 51i of the louver 51L. This is because, when inner wall 51i of louver 51L is curved in a dome shape as shown in FIG. 2, the portions defining inner wall 51i are approximated by straight lines.

[0036] On the other hand, the results of the fluid analysis revealed that as the mounting angle F becomes smaller, the responsiveness tends to decrease. Therefore, it was discovered that under the condition of F<α, which ensures water resistance as described above, if β≦F, both water resistance and responsiveness can be improved.

[0037] Specifically, this fluid analysis involves constructing a three-dimensional model of the sensor element 21, the metal shell 11, and the protector 51 with various mounting angles of the louvers 51L in the gas sensor 1 as shown in Figure 1, and using a turbulence model of computational fluid dynamics simulation to determine the state of gas replacement inside the protector 51 when the gas sensor 1 is attached to an exhaust pipe and air at 25°C is allowed to flow through it at a flow rate of 5 m / s. Inlet gas conditions: gas flow velocity 5 m / s, gas properties are those of 25°C air (density, viscosity), turbulence model: RANS k-ε model, analysis software: STAR-CCM+. Gas replacement is performed as a steady-state single-phase flow analysis in which a passive scalar substance (with a defined O2 diffusion coefficient) is introduced from the exhaust pipe inlet at time 0 seconds, and the replacement rate of the passive scalar substance on the surface of the detection part 22 at the tip of the sensor element 21 is extracted over time.

[0038] As shown in FIG. 4, when the gas replacement time T is T×1.2 when the installation angle of the louver 51L is 70 degrees, the second virtual installation angle of the louver 51L is β (where β<α), and it is preferable that β≦F<α. Here, the horizontal dashed line in Figure 4 indicates the time when T x 1.2 is achieved. The larger the installation angle of the louver 51L, the shorter the gas replacement time T, but T saturates at an installation angle of approximately 70 degrees, so the gas replacement time when the installation angle is 70 degrees was used as the standard. Since there is no practical problem even if the gas replacement time increases to (T x 1.2), the second virtual installation angle of the louver 51L when it is T x 1.2 was set to β.

[0039] In the above-mentioned fluid analysis, β=47 degrees and α=58 degrees. Therefore, it is preferable to set T to less than 58 degrees. It is even more preferable to set T to 47 degrees or more and less than 58 degrees.

[0040] The present invention is not limited to the above-described embodiments. The sensor element is not limited to one that measures the concentration of oxygen, but may be one that measures the concentration of nitrogen oxides (NOx) or hydrocarbons (HC), etc. The sensor element is not limited to a plate-like shape, but may be cylindrical. The shape and number of the gas inlet holes are not limited, and may be, for example, elliptical. The shape of the gas outlet holes is also not limited. [Explanation of symbols]

[0041] 1 Gas sensor 11 Metal body 21 Sensor element 22 Detection unit 51 Protector 51s First Perimeter Wall 51t Second wall 51f1 Tip-facing surface 51f2 Bottom 51x Large diameter section 51y small diameter section 51L Louver 51i Radial outer surface of louver (inner wall) 56 Gas inlet O axis G Center of gravity of protector V Virtual line

Claims

1. a sensor element extending in an axial direction and having a detection portion formed at a tip end thereof for detecting a target gas; a cylindrical metallic shell that surrounds and holds the sensor element in a radial direction; a single protector fixed to a tip end side of the metallic shell; Equipped with the protector includes a large diameter portion having a cylindrical first peripheral wall surrounding the tip end side of the sensor element and a tip-facing surface connected to the tip end side; a small diameter portion that protrudes from the large diameter portion toward a tip end in the axial direction and has a cylindrical second peripheral wall that is connected to the tip-facing surface and a bottom surface that is connected to the tip end side of the small diameter portion; a plurality of gas introduction holes formed at intervals in the circumferential direction on the tip-facing surface, a gas sensor in which an outer diameter of the first peripheral wall is larger than an outer diameter of the second peripheral wall, a louver extending toward a rear end side is provided on a radially inner side of the gas introduction hole, When a specific cross section that divides one louver equally in the circumferential direction is viewed along the axial direction and passes through the center of gravity of the protector, a virtual line that passes through the tip of the sensor element and is perpendicular to the axial direction intersects with the inner surface of the first peripheral wall at a point P, and a line segment that connects a tip Q of the radially outer surface of the louver to the intersection point P is defined as L, and the angle that the line segment L makes with the tip-facing surface is defined as a virtual mounting angle α. A gas sensor characterized in that the actual mounting angle F of the louver is less than α.

2. 2. The gas sensor according to claim 1, a three-dimensional model of the sensor element of the gas sensor, the metallic shell, and the protector with variously changed mounting angles of the louver is constructed, and the state of gas replacement within the protector when the gas sensor is attached to an exhaust pipe and air at 25°C is allowed to flow at a flow rate of 5 m / s is determined using a turbulence model of a computational fluid dynamics simulation; Here, at time 0 seconds, O ２ When a passive scalar material with a defined diffusion coefficient is introduced and a steady-state single-phase flow analysis is performed to extract the replacement rate of the passive scalar material at the tip of the sensor element over time, When the gas replacement time T is T×1.2 when the installation angle of the louver is 70 degrees, the second virtual installation angle of the louver is β (where β<α), A gas sensor characterized in that the mounting angle F of the louver is equal to or larger than β and smaller than α.

3. 2. The gas sensor according to claim 1, wherein the mounting angle F of the louver is less than 58 degrees.

4. 3. The gas sensor according to claim 2, wherein the louver has an angle F of 47 degrees or more and less than 58 degrees.

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