Gas sensor

Abstract

Gas sensor comprehensive: a cylindrical housing (H); a sensor element (1) which is inserted into and held in the cylindrical housing (H), wherein the sensor element (1) has a detection section (3) at a distal end region of the same for detecting a specific gas concentration in a measured gas (G), and an element cover (2) arranged at a distal end side of the cylindrical housing (H), which surrounds a periphery of the sensor element (1) projecting from the cylindrical housing (H); wherein the sensor element (1) comprises: a solid electrolyte body (11) with oxygen ion conductivity, wherein the solid electrolyte body (11) is formed in a cylindrical shape; Electrodes (31, 32) formed on a surface of the solid electrolyte body (11), wherein the electrodes (31, 32) form the detection section (3); a heating element (4) housed in a cylinder of the solid electrolyte body (11); and a porous protective layer (5) formed on an outer surface of the detection section (3); wherein the element cover (2) has a double cover structure formed from an inner cover (21) and an outer cover (22), wherein a space is formed between the distal end surfaces of the inner cover (21) and the outer cover (22); the inner cover (21) is provided with gas flow openings (21a) on a side surface of the inner cover (21), and a gas passage (23) is formed between an outer surface of the sensor element (1) and an inner surface of the inner cover (21); and a distance D between the inner cover (21) and the side surface of the sensor element (1) in a direction perpendicular to an axis of the sensor element (1) in the entire area of ​​the gas passage leading from the gas flow openings (21a) to the detection section (3) lies in a range of 0.2 mm to 0.8 mm, and a space over an entire part of a distal end face in an axial direction of the inner cover (21), which overlaps with the outer cover (22), is formed such that the distal end surface of the inner cover (21) is not connected to the distal end surface of the outer cover (22).

Description

BACKGROUND OF THE INVENTION Technical field of the invention

[0001] The present invention relates to a gas sensor for measuring a specific gas concentration in a measured gas. DESCRIPTION OF THE STATE OF THE TECHNOLOGY

[0002] A gas sensor for measuring an oxygen concentration or the like in an exhaust gas is arranged in an exhaust system of an internal combustion engine.

[0003] In general, the gas sensor is attached to an exhaust pipe wall in a state in which a sensor element, on which a pair of electrodes are formed on a surface of a solid electrolyte body with oxygen ion conductivity, is inserted into a housing and protected by an element cover attached to a distal end of the housing.

[0004] A gas sensor, the sensor element of which is protected by covering an outer surface of a distal end part of a laminated sensor element with a porous protective layer of 10 µm or more thickness, and in which a free space between the porous protective layer and the element cover is defined to a predetermined area so that the activation time can be reduced, is disclosed in Japanese patent application JP 2009 - 80 100 A.

[0005] The free space between the porous protective layer and the element cover is defined such that 35% or more of a total circumference, for example, is 1 to 4 mm, so that both control of the thermal transfer through the element cover and the retention of heat through radiant heat can be achieved, thereby maintaining a detection section at a desired temperature.

[0006] On the other hand, depending on the operating condition of an internal combustion engine, particulate material (i.e. PM) such as soot may be present in the exhaust gas.

[0007] Furthermore, water may condense in the exhaust pipe at the time the engine is started.

[0008] Therefore, the element cover of the gas sensor is designed to have, for example, a double structure with an outer cover and an inner cover, and by working out the size and arrangement of the gas flow openings of each of the covers, PM and condensed water in the exhaust gas are prevented from entering the element cover.

[0009] However, it is impossible to completely prevent PM and condensed water from penetrating. In particular, PM can simply adhere to and deposit in a relatively low-temperature area, e.g., on an inner circumferential surface of the inner cover within the element cover.

[0010] If there is an area where part of the free space between the sensor element and the inner cover is small, as disclosed in JP 2009 - 80 100 A, the distribution of the exhaust gas will be blocked if the PM is deposited in the free space and there is a possibility that the sensor detection accuracy will be reduced.

[0011] US 2002 / 0100687A1 discloses a gas sensor comprising a sensor element for detecting a component of a gas and a cup-shaped protector for covering a detection area of ​​the sensor element. The protector is double-walled and has cup-shaped inner and outer protective elements. The inner and outer protective elements are designed and arranged such that g1 > g2, where g1 is the distance between an inner surface of a circumferential wall of the outer protective element and an outer surface of a circumferential wall of the inner protective element, and g2 is the distance between an inner surface of a bottom wall of the outer protective element and an outer surface of a bottom wall of the inner protective element. The outer protective element has a plurality of first gas openings on its circumferential wall. The inner protective element has a plurality of second gas openings on its circumferential wall.If the sensor element is oriented "forward" at the tip, the first gas holes are located in front of the second gas holes. The front end of the sensor element is positioned in front of the second gas holes. The inner and outer protective elements have gas holes on their lower walls. SUMMARY OF THE INVENTION

[0012] The present invention was made in light of the problems outlined above and its aim is to provide a gas sensor that is able to prevent the flow of exhaust gas from being blocked due to particulate material that penetrates and is deposited between a sensor element and an element cover, thereby maintaining the sensor detection accuracy.

[0013] The objective is achieved by the gas sensor with the features of claim 1 and the gas sensor with the features of claim 7. Further advantageous embodiments are the subject of the subsequent claims.

[0014] In a gas sensor according to a first aspect, the gas sensor comprises a cylindrical housing, a sensor element which is inserted and held in a cylindrical housing, with a sensing section at a distal end region of the same for sensing a specific gas concentration in a measured gas, and an element cover which is arranged at a distal end side of the housing which surrounds a periphery of the sensor element protruding from the housing.

[0015] The sensor element comprises a solid electrolyte body with oxygen ion conductivity, electrodes formed on a surface of the solid electrolyte body formed in a cylindrical shape, the electrodes forming the sensing section, a heating element housed in a cylinder of the solid electrolyte body, and a porous protective layer formed on an outside of the sensing section.

[0016] The element cover comprises a double-cover section consisting of an inner cover and an outer cover, with a space formed between the distal end surfaces of the inner and outer covers. The inner cover is provided with gas flow openings on one side surface, and a gas passage is formed between an outer surface of the sensor element and an inner surface of the inner cover.

[0017] The distance between the inner cover and the side surface of the sensor element in a direction perpendicular to an axis of the sensor element lies in a range of 0.2 mm to 0.8 mm throughout the entire area of ​​the gas passage leading from the gas flow openings to the detection section.

[0018] A space is formed over an entire part of a distal end face in an axial direction of the inner cover, overlapping with the outer cover, such that the distal end surface of the inner cover is not connected to the distal end surface of the outer cover.

[0019] The measured gas flows through the gas flow openings into the cover part that forms the element cover, flows through the porous protective layer at the distal end of the sensor element and reaches the electrodes of the detection section.

[0020] At this time, the heating element is housed in the tubular solid electrolyte body, so that the entire sensor element can be heated evenly.

[0021] Furthermore, the heat from the sensor element can be efficiently absorbed by the cover part wall, since the distance D between the sensor element and the cover part in the direction perpendicular to the axis in the entire gas passage leading from the gas flow openings to the detection section is fixed to a predetermined small free space.

[0022] This prevents the accumulation of particulate material, even if the particulate material contained in the measured gas penetrates the element cover, as any particulate material adhering to the passage wall is burned and removed from the gas passage opposite the sensing section of the sensor element.

[0023] As previously described, according to the preceding aspect, particulate material is prevented from penetrating and accumulating between a sensor element and an inner cover, thereby maintaining high sensor detection accuracy. BRIEF DESCRIPTION OF THE FIGURES

[0024] The accompanying characters show: Fig. 1 a longitudinal sectional view of a main area of ​​a gas sensor according to a first embodiment; Fig. Figure 2 shows a cross-sectional view of a main area of ​​the gas sensor, wherein a sectional view according to the first embodiment is shown, extending along a line II-II in Fig. 1 has been taken; Fig. Figure 3 shows a longitudinal section view of the overall structure of the gas sensor according to the first embodiment; Fig.Figure 4 shows a schematic view to explain a test procedure that uses a test apparatus according to a first test example; Fig. Figure 5 shows a graph of the relationship between a distance D, between a sensor element and an inner cover in a direction perpendicular to an axis, and a PM combustion rate according to the first test example; and Fig. Figure 6 shows a graph of a relationship between the distance D, between the sensor element and the inner cover in the direction perpendicular to the axis and the temperature T of the inner cover according to a second test example. DETAILED DESCRIPTION OF THE PREFERRED EXECUTIONS [First embodiment]

[0025] The following describes embodiments of the invention with reference to Fig. 1, Fig. 2 to Fig. 3 described.

[0026] As in Fig.Figure 1 shows a gas sensor S of a first embodiment comprising a sensor element 1 which is inserted into and held in a cylindrical housing H and an element cover 2 which is arranged on a distal end side of the housing H.

[0027] The sensor element 1 defines a vertical direction in the figure as an axial direction X and a direction perpendicular to it as a direction Y perpendicular to an axis.

[0028] The sensor element 1 has a detection section 3 at a distal end region of the same (i.e. a lower end region of the figure) for detecting a specific gas concentration in a measured gas and a heating element 4 therein.

[0029] The element cover 2 is arranged to surround and protect a periphery of the sensor element 1 that protrudes from the housing H.

[0030] The gas sensor S is installed, for example, in an exhaust pipe of an internal combustion engine and can be used as an exhaust gas sensor to detect a specific gas concentration in the measured exhaust gas, such as an air-fuel ratio sensor or an oxygen sensor.

[0031] According to an exemplary design of the air-fuel ratio sensor, as it is used in Fig. Figure 3 is shown as an example, the gas sensor S is attached to an exhaust pipe wall (not shown) by a screw area provided at a distal end area of ​​an outer periphery of the housing H.

[0032] A central area with a larger diameter of the sensor element 1 is held by a stepped area provided in a cylinder of the housing H.

[0033] Furthermore, a cylindrical insulating material I is filled between a base end face (i.e., upper end face in the figure) of an outer circumferential surface of the sensor element 1 and an inner circumferential surface of the housing H.

[0034] A base end region (i.e., upper end region in the figure) of the sensor element 1 protrudes from the housing H and is housed in an atmospheric side cover 6.

[0035] The tubular atmospheric side cover 6 is attached to the base end of the housing H and is adapted to receive air from the inlet openings 61 which open on an outer circumferential side surface of the same.

[0036] The sensor element 1 comprises a solid electrolyte body 11 which is formed in a cylindrical shape with a base, the detection section 3 with measuring electrodes 31 and a reference electrode 32 which is arranged at a distal end region of the solid electrolyte body 11 and a porous protective layer 5 which covers the distal end region of the solid electrolyte body 11.

[0037] The porous protective layer 5 covers a surface of the distal end region of the sensor element 1 which is located inside the element cover 2 and exposed to the exhaust gas, thereby protecting the sensor element 1 from the water.

[0038] In the present embodiment, the porous protective layer 5 has a two-layer structure consisting of a trapping layer 51 and a diffusion layer 52.

[0039] The porous protective layer 5 can be designed as a single layer or as a multi-layer structure consisting of three or more layers.

[0040] The rod-shaped heating element 4 is mounted coaxially inside a cylinder made of the solid electrolyte body 11.

[0041] By being supplied with energy, the heating element 4 generates heat and heats the detection section 3 to a temperature equal to or higher than an activation temperature (e.g. 500°C).

[0042] The solid electrolyte body 11 is a sintered ceramic body, which is formed, for example, from partially stabilized zirconium dioxide or the like and exhibits hydrogen ion conductivity.

[0043] A base end region of the solid electrolyte body 11 is open towards the interior of the atmosphere side cover 6 and an atmosphere is introduced into the solid electrolyte body 11 as a reference gas.

[0044] A plurality of connecting wires S1 to S3, which are connected to a control unit (not shown), are insulated and held at a base end opening area of ​​the atmosphere side cover 6 and are electrically connected to a plurality of connection areas T1 to T3, which extend from the sensor element 1 into the atmosphere side cover 6.

[0045] The connecting wires S1, S2, and the connection areas T1, T2 are each connected to the measuring electrodes 31 and the reference electrode 32 of the detection section 3.

[0046] The connecting wire S3 and the connection area T3 are connected to the heating element 4.

[0047] In Fig. 1 and Fig. 2 A distal end region of a closed end side of the sensor element 1 protrudes into the element cover 2 and is positioned coaxially.

[0048] The measuring electrodes 31 with a predetermined width are formed on an outer surface of the solid electrolyte body 11 near the distal end of the sensor element 1.

[0049] The reference electrode 32 is formed on an inner circumferential surface of the solid electrolyte body 11, which is opposite the measuring electrodes 31, so that the solid electrolyte body 11 lies between them.

[0050] The measuring electrode 31 of the detection section 3 is exposed to the exhaust gas G, which is introduced into the element cover 2, and the reference electrode 32 is exposed to the atmosphere, which is introduced into the solid electrolyte body 11, whereby the oxygen concentration is detected on the basis of a potential difference generated between the electrodes.

[0051] The surface of the distal end region of the sensor element 1, which is located within the element cover 2, is covered with the diffusion layer 52 and the interception layer 51 is formed on the outside of the diffusion layer 52.

[0052] The intercepting layer 51 is a porous layer made of a ceramic material such as aluminium dioxide, with good resistance to thermal shocks and protects the sensor element 1 by intercepting the condensed water in the exhaust gas G.

[0053] The porosity of the intercepting layer 51 is, for example, 50% to 90%, and a heat transfer effect described later becomes pronounced when the porosity is high and the thermal capacity is low, simply causing the diffusion of the adhering condensed water.

[0054] The diffusion layer 52 is made of a ceramic layer of spinel, etc., and while it protects the surface of the sensor element 1, the diffusion layer 52 transmits the exhaust gas G at a predetermined diffusion resistance.

[0055] The porosity or thickness of the diffusion layer 52 is adjusted to achieve a desired diffusion resistance, and typically its porosity is lower than that of the intercepting layer 51.

[0056] In the present embodiment, the element cover 2 is a double-cover structure and is formed by an inner cover 21 with a small diameter and an outer cover 22 with a larger diameter than the inner cover 21, both having a cylindrical shape with a bottom.

[0057] A plurality of gas flow openings 21a are formed on a side surface of the base end face of the inner cover 21 as a cover part and a plurality of gas flow openings 21b are formed on the distal end surface of the inner cover 21.

[0058] A plurality of gas flow openings 22a are formed on a side surface of the base end face of the outer cover 22, which is arranged coaxially on an outside of the inner cover 21, and a plurality of gas flow openings 22b are formed on the distal end surface of an outer periphery of the outer cover 22.

[0059] Although the axial length of the inner cover 21 is shorter than the axial length of the outer cover 22, and the relatively large gas flow openings 22a are formed on the side surface of the outer cover 22, which faces a space formed between the distal end surfaces of the two covers, this does not necessarily imply a restriction to the configuration shown in the figures.

[0060] Furthermore, the element cover 2 can also be a single cover structure that does not have an outer cover 22, or a triple or multiple cover structure with a plurality of cover parts outside the inner cover 21.

[0061] An inner gas passage 23 is formed between an inner surface of the inner cover 21a and an outer surface of the sensor element 1.

[0062] Furthermore, an external gas passage 24 is formed between an outer surface of the inner cover 21 and an inner surface of the outer cover 22.

[0063] The inner cover 21 has a cylindrical shape, except for a base end region which is attached to the housing together with the outer cover 22, constant diameter and a distal end region with the gas flow openings 21b.

[0064] Furthermore, an annular gas passage with a constant width is formed between the inner cover 21 and the outer surface of the sensor element 1, which has a cylindrical shape and a constant diameter.

[0065] Similarly, an annular gas passage with a constant width is formed between the outer surface of the inner cover 21 and the inner surface of the outer cover 22, which has a cylindrical shape and constant diameter.

[0066] The exhaust gas G is introduced from the gas flow openings 22a at the distal end of the outer cover 22 into the outer gas passage 24 and flows to the opposite gas flow openings 22a.

[0067] Some of the exhaust gas G flows into the annular gas passage, which extends to the gas flow openings 21a, and further flows from the gas flow openings 21a into the interior of the gas passage 23 and flows in the direction of the detection section 3.

[0068] Depending on the combustion state of the combustion engine, particulate material from the fuel may be contained in the exhaust gas G at this time, and if the particulate material adheres to and is deposited in a low-temperature area of ​​the element cover 2, the flow of the exhaust gas G is blocked.

[0069] In particular, the inner gas passage 23 tends to have a lower temperature than the outer gas passage 24, whose outer cover 22 is exposed to the exhaust gas G at high temperature in the exhaust pipe, since it is positioned on the inside of the element cover 2.

[0070] Furthermore, water may be present in the exhaust pipe at the time of starting the engine, and this, together with the exhaust gas G, simply reaches the surface of the sensor element 1, which means that cracks may occur when the water is heated.

[0071] Therefore, with regard to the inner gas passage 23, the particulate material adhering to the passage wall can be burned and removed by the heat of the heating element 4 by reducing the distance between the opposing surfaces of the annular gas passages.

[0072] In the annular gas passages extending from the gas flow openings 21a of the inner gas passage 23 to the detection section 3, a distance D in a direction perpendicular to an axis (hereinafter simplified as distance D) between the outer surface of the sensor element 1, i.e., the surface of the intercepting layer 51, which serves as an outermost layer of the porous protective layer 5, and the inner surface of the inner cover 21, which is opposite the outer surface of the sensor element 1, is designed such that it lies in the range of 0.2 mm to 0.8 mm throughout the entire area.

[0073] A gap required for assembly can be achieved by configuring the gap D to 0.2 mm or more.

[0074] Furthermore, the inner surface of the inner cover 21 can easily absorb heat by configuring the distance D to be 0.8 mm or less, thus enabling the inner surface of the inner cover 21 to be heated above the combustion temperature of the particulate material (e.g. 500°C).

[0075] Preferably, the distance D can be in the range of 0.2 mm to 0.6 mm across the entire area.

[0076] Here, the annular gas passage extending from the gas flow openings 21a to the detection section 3 is defined as a passage at least from a position of a distal end-side edge region of the gas flow opening 21a to a position of a base end-side edge region of the measuring electrode 31 of the detection section 3.

[0077] By preventing the particulate material from accumulating in the annular passageways in this area, the exhaust gas G can sufficiently reach the measuring electrode 31, thus maintaining the detection accuracy.

[0078] As shown in the figures, the distance D between the total surface of the measuring electrode 31 and the inner surface of the inner cover 21 and furthermore in the annular passage around the sensing section 3 including a position in the distal end face of the measuring electrode 31 may preferably be in the aforementioned area.

[0079] For the outer gas passage 24, the distance between the opposing surfaces of the annular gas passages is sufficiently larger than that of the inner gas passage 23, so that it does not impede the flow of the exhaust gas G.

[0080] In particular, the exhaust gas G can flow sufficiently, even if the particulate material adheres to the passage wall.

[0081] A distance between the outer surface of the inner cover 21 and the inner surface of the outer cover 22 in the direction perpendicular to the axis is, for example, designed such that it lies in a range of 1 mm or more for the entire area of ​​the annular gas passage which extends to the gas flow openings 21a of the inner gas passage 23.

[0082] Preferably, it is appropriately defined, for example, by a shape or an outer diameter of the element cover 2 in the range of 1 mm to 6 mm.

[0083] Preferably, by adjusting the thickness of the intercepting layer 51 or the diffusion layer 52, which serve as the porous protective layer 5, both the adjustment of the distance D between the outer surface of the sensor element 1 and the inner surface of the inner cover 21 and protection from water can be achieved.

[0084] In particular, since the free space to the opposite inner surface of the inner cover 21 decreases by making the porous protective layer 5 thicker, the distance D can thus be easily designed within the aforementioned area.

[0085] The total thickness of the porous protective layer 5 comprising the interception layer 51 and the diffusion layer 22 is preferably in a range of 500 µm to 1200 µm.

[0086] If the thickness is less than 500 µm, there are concerns that the effect of preventing breakage of the element and heat transfer due to condensed water cannot be sufficiently achieved, and if the thickness exceeds 1200 µm, there are concerns that the clearance to the inner cover 21 will be reduced, thus reducing the ease of assembly.

[0087] At this time, the thickness of the diffusion layer 52 and the interception layer 51 is designed, for example, in the range of 250 µm to 600 µm, and the total thickness of the porous protective layer 5 which encompasses them is suitably determined so that it lies in the aforementioned range of 250 µm to 600 µm.

[0088] For example, it is easy to determine the distance D if the thickness of the intercepting layer 51, which serves as the outermost layer of the porous protective layer 5, is adjusted.

[0089] If the thickness of the intercepting layer 51 is less than 250 µm, the effect of intercepting the condensed water and heat transfer cannot be sufficiently achieved, and if the thickness of the intercepting layer 51 is increased, more condensed water can be intercepted, so that it is possible to protect the capture section 3.

[0090] At this time, since the heat capacity is low and the porosity is high in the intercepting layer 51, the heat from the heating element 4, which is integrated into the sensor element 1, is efficiently transferred to the inner surface of the opposite inner cover 21.

[0091] As previously described, an embodiment is used in which the rod-shaped heating element 4 is housed in the cylindrical sensor element 1.

[0092] Accordingly, the inner surface of the inner cover 21 is heated uniformly by heat conduction from the sensor element 1 and can be kept at a high temperature by setting the distance D between the inner cover 21 under the element cover 2 and the sensor element 1 or the thickness of the porous protective layer 5 formed on the surface of the sensor element 1 within the predetermined range.

[0093] Thus, the adhering particulate material can be burned and removed, even if the particulate material, along with the exhaust gas G, enters the inner gas passage 23 in the inner cover 21.

[0094] The particulate material can be prevented from accumulating in the outer gas passages 24 within the outer cover 22 by further shaping the ring-shaped gas passages.

[0095] As a result, the detection accuracy of sensor element 1 can be kept high without blocking the flow of exhaust gas G.

[0096] Furthermore, the sensor element is protected from a large amount of condensed water at the time of start-up by making the intercepting layer generated from the porous protective layer 5 thick, which makes it possible to increase the effect of preventing the element from breaking.

[0097] The gas sensor S with the aforementioned structure can be used as a fuel-air ratio sensor or an oxygen sensor, which is arranged upstream or downstream of an exhaust gas purification catalyst, which is mounted in an exhaust pipe in an exhaust gas purification system of a motor vehicle engine, which is an internal combustion engine.

[0098] In such a system, there is usually a risk that small amounts of particulate material may interfere with the output signal, since feedback control is based on the output signal of the air-fuel ratio sensor, which is located on a downstream side.

[0099] For this reason, an air-fuel ratio sensor is needed that enables high detection accuracy and good response by reliably removing the particulate material.

[0100] By using the gas sensor S of the present embodiment, it is even possible in such a case to begin the detection of the air-fuel ratio by rapidly raising the temperature to the activation temperature while the engine is being started, thus making it possible to improve engine controllability. [1. Test example]

[0101] By using a test apparatus 100, which is in Fig. As shown in Figure 4, combustion effects of the particulate material were investigated in the gas sensor S with the aforementioned configuration when the distance D between the inner cover 21 and the sensor element 1 was changed.

[0102] The test apparatus 100 comprises a line 102 which is connected to a hot air device 101 and the apparatus 100 is designed such that hot air at a predetermined temperature, corresponding to the exhaust gas of a motor vehicle engine, can be supplied to the line 102 at a predetermined flow rate.

[0103] Here, the temperature of the hot air is set to 400°C, the flow rate is set to 5 l / min, and the distance D was varied in a range of 0.2 mm to 1.3 mm.

[0104] The housing H of the gas sensor S is screwed into the line 102 wall and the element cover 2, which is located at the distal end of the housing A, protrudes into and is positioned in the line 102.

[0105] Prior to the test, the particulate material that had been collected beforehand was held adhering to the inner surface of the inner cover 21 of the element cover 2 in the gas sensor S.

[0106] Then, after measuring the weight of the gas sensor S equipped with the element cover 2, the gas sensor S was attached to the test apparatus 100.

[0107] The mounting position of the gas sensor S was set at a position where a distance D from an upstream end of line 102 is 0.3 m.

[0108] The hot air was introduced into line 102 by the hot air device 101 by operating the test apparatus 100d and the gas sensor S was operated by a control unit (not shown) for 10 minutes.

[0109] Subsequently, the gas sensor S was removed from line 102, the weight of the gas sensor S was measured again, and the combustion rate of the adhering particulate material (i.e., PM combustion rate) was calculated based on the weight difference between before and after the test.

[0110] The results are in Fig. 5 shown.

[0111] As demonstrated by Fig. As can be seen in Figure 5, the PM combustion rate is high when the distance D between the inner cover 21 and the sensor element 1 is small, which is approximately 83% at 0.2 mm.

[0112] When the distance D becomes large, the PM combustion rate tends to gradually decrease.

[0113] However, the PM combustion rate is approximately 80% when the distance D is 0.6 mm, which is not a significant reduction, and a PM combustion rate of more than approximately 75% can be maintained even when the distance D is 0.8 mm.

[0114] Then, when the distance D exceeds 0.8 mm, the PM combustion rate drops sharply; when the distance D exceeds 1 mm, the rate is approximately 60%; and when the distance D is 1.3 mm, the rate drops to approximately 48%.

[0115] Thus, by arranging the inner cover 21 relative to the sensor element 1, the distance D 02 mm becomes 0.8 mm (range b in Fig. 5), preferably 0.2 mm to 0.6 mm (range a in Fig. 5) It can be seen that, even if the particulate material adheres, it can be burned and removed. [Second test example]

[0116] In the gas sensor S with the aforementioned configuration, another test is carried out in the same way as in the first test example by producing two size types of the solid electrolyte body 1 of the sensor element 1 (i.e., element size: φ 4.5 mm and φ 3.9 mm), and changing the gas flow rate of the test apparatus 100 in a range of 1 to 10 l / min.

[0117] The distance D between the inner cover 21 and the sensor element 1 is set in three ways, namely 0.6 mm, 0.8 mm, and 1.0 mm.

[0118] The results are shown in Table 1.

[0119] The PM combustion rate was assessed in three steps: A: 80% or more, B: 75% or more, and C: less than 75%.

[0120] Furthermore, the temperature of the inner surface of the inner cover 21 was measured, and a relationship between the temperature and the distance D was found. Fig. 6 shown. Element size (φ ZrO2) Gas flow rate (L / min) Element-Inner Cover-Distance D (mm) PM combustion rate φ 4.5 1 0,6 A 0,8 B 1 C 5 0,6 A 0,8 B 1 C 10 0,6 A 0,8 A 1 C φ 3.9 1 0,6 A 0,8 B 1 C 5 0,6 A 0,8 B 1 C 10 0,6 A 0,8 A 1 C A: 80% or more B: 75% or more C: less than 75% [Table 1]

[0121] As can be seen from Table 1, there is a tendency for the PM combustion rate to increase the smaller the distance D is and the higher the gas flow rate.

[0122] If the distance D is 0.6 mm, a PM combustion rate of 80% or more can be achieved, regardless of the element size and gas flow rate.

[0123] If the distance D is 0.8 mm, a PM combustion rate of 80% or more can be achieved at a gas flow rate of 10 L / min, otherwise the PM combustion rate is 75% or more.

[0124] If, however, the distance D is 1.0 mm, a PM combustion rate of 75% or more cannot be achieved, even if the exhaust gas flow rate is increased.

[0125] Furthermore, the temperature of the inner cover 21 is related to the temperature in Fig.6 shown D and when the distance D becomes smaller, the temperature increases due to heat absorption by the heating element 4 integrated in the gas sensor S.

[0126] Particularly in the area where the distance D is 0.8 mm or less, the temperature is 500°C or more, meaning the temperature is higher than required to burn the particulate material; the results are consistent with the results in Table 1 where a high PM combustion rate is achieved. [Third test example]

[0127] Next, the presence or absence of fractures in the solid electrolyte body 11 of the sensor element 1 in the gas sensor S of the aforementioned construction was investigated by performing a water test using the following procedure when the thickness of the porous protective layer 5 including the intercepting layer 51 and the diffusion layer 52 was changed.

[0128] Prior to the water test, an exposed distal end area of ​​the sensor element 1 was created by cutting off the element cover 2 of the gas sensor S, and the gas sensor S was operated in this state.

[0129] A drop of water with a volume of 30 microliters was dropped onto the detection section 3 of the gas sensor S, and the presence or absence of fractures in the solid electrolyte body 11 was then confirmed by visual inspection.

[0130] The thicknesses of the porous protective layers 5 are designed to range from 200 micrometers to 1,300 micrometers, and a plurality of samples for each thickness were produced.

[0131] The results are shown in Table 2.

[0132] Here, the number of samples is set to 4, and the number of fractures found in the sample is shown in the table. [Table 2] Thickness of the porous protective layer (mm) Presence or absence of fractures in the solid electrolyte body after the water test* 200 4 / 4 300 3 / 4 500 0 / 4 700 0 / 4 800 0 / 4 1.000 0 / 4 1.200 0 / 4 1.300 - * Number of fractures / Number of samples

[0133] As can be seen from Table 2, the thicker the porous protective layer 5 is, the fewer fractures occur due to the water, and when the thickness of the porous protective layer 5 is 500 micrometers or more, no samples were fractured.

[0134] However, when the thickness of the porous protective layer was 5,200 micrometers, fractures were observed in all samples, and at 300 micrometers there was only one sample with no fracture.

[0135] Furthermore, there is a possibility that the sensor element 1 and the element cover 2 in the sample come into contact with the porous protective layer 5 with a thickness of 130 micrometers, and it was therefore concluded that it was impossible to assemble them.

[0136] Accordingly, the effect of preventing the gas sensor S from breaking due to water is strong when the thickness of the porous protective layer 5 is in the range of 500 micrometers to 1,200 micrometers.

[0137] The gas sensor S is not limited to the configuration of the aforementioned embodiment; however, various modifications are possible within the scope of the present invention.

[0138] For example, the structure of the detection section 3 of the sensor element 1, the shape of the element cover 2 to protect the sensor element 1, the size and number of the gas flow openings, the arrangement and the like can be configured as desired.

[0139] Even though the gas sensor S is used as an example in the aforementioned embodiment as the air-fuel ratio sensor or the oxygen sensor, which are mounted in the exhaust pipe of the motor vehicle engine, other gases in the exhaust gas can be a detection target.

[0140] Furthermore, the present invention can be applied to various uses apart from the exhaust gas sensor, and the measured gas is not to be understood as the exhaust gas from the motor vehicle engine, but the measured gas can be a combustion exhaust gas or the like, comprising particulate material with oxygen and other gases to be detected.

Claims

Gas sensor comprising: a cylindrical housing (H); a sensor element (1) inserted into and held in the cylindrical housing (H), the sensor element (1) having a sensing section (3) at a distal end region thereof for sensing a specific gas concentration in a measured gas (G); and an element cover (2) arranged at a distal end side of the cylindrical housing (H), which surrounds a periphery of the sensor element (1) projecting from the cylindrical housing (H); wherein the sensor element (1) comprises: a solid electrolyte body (11) with oxygen ion conductivity, the solid electrolyte body (11) being formed in a cylindrical shape; electrodes (31, 32) formed on a surface of the solid electrolyte body (11), the electrodes (31, 32) forming the sensing section (3); a heating element (4) housed in a cylinder of the solid electrolyte body (11);and a porous protective layer (5) formed on an outer surface of the sensing section (3); wherein the element cover (2) has a double-cover structure formed from an inner cover (21) and an outer cover (22), wherein a space is formed between the distal end surfaces of the inner cover (21) and the outer cover (22); the inner cover (21) is provided with gas flow openings (21a) on a side surface of the inner cover (21), and a gas passage (23) is formed between an outer surface of the sensor element (1) and an inner surface of the inner cover (21);and a distance D between the inner cover (21) and the side surface of the sensor element (1) in a direction perpendicular to an axis of the sensor element (1) in the entire area of ​​the gas passage leading from the gas flow openings (21a) to the detection section (3) is in a range of 0.2 mm to 0.8 mm, and a space over an entire part of a distal end surface in an axial direction of the inner cover (21), which overlaps with the outer cover (22), is formed such that the distal end surface of the inner cover (21) is not connected with the distal end surface of the outer cover (22). Gas sensor according to claim 1, wherein the distance D in the direction perpendicular to the axis is in a range of 0.2 to 0.6 mm. Gas sensor according to claim 1 or 2, wherein the thickness of the porous protective layer (5) is in a range of 500 µm to 1,200 µm. Gas sensor according to one of claims 1 to 3, wherein the outer cover (22) is arranged coaxially on an outer surface of the inner cover (21) and is provided with gas flow openings (22a) on a side surface of the outer cover (22); a gas passage (24) is formed between the outer cover (22) and the surface of the inner cover (21); and the heating element (4) heats the inner surface of the inner cover (21) to a temperature of 500°C via the porous protective layer (5). Gas sensor according to one of claims 1 to 4, wherein the measured gas is an exhaust gas emitted by an internal combustion engine and comprises particulate material. Gas sensor according to claim 5, wherein the gas sensor is mounted such that the element cover protrudes into and is positioned in the exhaust gas downstream side of a catalyst arranged in the exhaust gas line of the internal combustion engine and detects an air-fuel ratio of the exhaust gas that has passed through the catalyst. Gas sensor comprising: a cylindrical housing (H); a sensor element (1) inserted into and held in the cylindrical housing (H), the sensor element (1) having a sensing section (3) at a distal end region of the same for sensing a specific gas concentration in a measured gas (G); and an element cover (2) arranged at a distal end side of the cylindrical housing (H), which surrounds a periphery of the sensor element (1) projecting from the cylindrical housing (H); wherein the sensor element (1) comprises: a solid electrolyte body (11) with oxygen ion conductivity, wherein the solid electrolyte body (11) is formed in a cylindrical shape; electrodes (31, 32) formed on a surface of the solid electrolyte body (11), wherein the electrodes (31, 32) form the sensing section (3); a rod-shaped heating element (4) housed in a cylinder of the solid electrolyte body (11);and a porous protective layer (5) formed on an outer surface of the detection section (3); wherein the element cover comprises a cover part (21) provided with gas flow openings (21a) on a side surface of the cover part (21), and a gas passage (23) is formed between an outer surface of the sensor element (1) and an inner surface of the cover part (21); a distance D between the cover part (21) and the side surface of the sensor element (1) in a direction perpendicular to an axis of the sensor element (1) in the entire area of ​​the gas passage leading from the gas flow openings (21a) to the detection section (3) is in a range of 0.2 mm to 0.8 mm, and the cover part (21) of the element cover forms an inner cover, wherein the element cover comprises an outer cover;and a space over an entire part of a distal end face in an axial direction of the inner cover, which overlaps with the outer cover, is formed such that the distal end surface of the inner cover is not connected to the distal end surface of the outer cover.

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