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(11) | EP 1 026 447 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Glow plug |
| (57) The metallic shell (20) of a glow plug has a cylindrical male-threaded portion (21)
and a tightening portion (22). The tightening portion (22) is formed to have an outer
circumferential surface of a shape corresponding to that of a 12-point nut. By virtue
of this structure, the tightening portion (22) can have a greater wall thickness as
compared with a conventional tightening portion which has a an outer circumferential
surface of a shape corresponding to that of a hexagon nut. Therefore, it becomes possible
to made the glow plug thinner, while maintaining the strength of the tightening portion
(22). |
[0002] Conventionally, a glow plug as shown in FIG. 6 has been known. FIG. 6 shows a longitudinal cross section of the glow plug. The glow plug, denoted by reference numeral 50, comprises a metallic shell 51. The metallic shell 51 has on its outer circumference a male-threaded portion 52, which is to be screw-engaged with a female-threaded hole formed in a head of a diesel engine. Above the male-threaded portion 52 is formed a tightening portion 53 with which a plug wrench is engaged. As shown in FIG. 7, which shows a plan view of the tightening portion 53, the tightening portion 53 is shaped to have an outer circumferential shape similar to that of a hexagon nut. Further, a tube 54 is inserted into the metallic shell 51 such that the tube 54 is exposed to a combustion chamber of the diesel engine.
[0003] A heating coil 55 for heating the tube 54 is accommodated within the tube 54 to be located at the tip end thereof. One end of the heating coil 55 is welded to the tip end of a center shaft 56, which is inserted into the metallic shell 51 and the tube 54. A rubber packing 57 is disposed at the upper end of a press-fitted portion of the tube 54 in order to secure the airtight integrity of the interior space of the metallic shell 51. An insulation member 58 is fitted onto an upper portion of the center shaft 56 such that the center shaft 56 penetrates the insulation member 58 and a projected portion of the insulation member 58 is fitted into the tightening portion 53. An O-ring made of rubber is disposed within a space defined by the lower surface of the insulation member 58, the inner wall surface of the tightening portion 53, and the center shaft 56, in order to secure the airtight integrity of the interior space of the metallic shell 51. Further, a round nut 60 is fitted onto a male thread 56a of the center shaft 56. When the round nut 60 is tightened, the insulation member 58 moves downward, so that the O-ring 59 is pressed and expanded radially. Consequently, the annular space between the insulation member 58 and the metallic shell 51 is closed completely to thereby secure the airtight integrity of the interior space of the metallic shell 51.
[0004] Incidentally, with progress in the development of diesel engines, the structure of a diesel engine, particularly the structure of that portion of the engine around an engine head, has become complicated. Therefore, a space on an engine head available for attachment of a glow plug has been decreased.
[0005] Thus, there has arisen a demand for a glow plug in which the tightening portion 53 to be located on the engine head has a reduced across-face dimension (also known as the width-across-flat or A/F). Especially, four-valve engines require a glow plug whose tightening portion 53 has a reduced across-face dimension, because such a four-valve engine has a more complicated structure around an engine head thereof and a further decreased area for attachment of a glow plug.
[0006] However, when the across-face dimension of the tightening portion 53 is decreased, the wall thickness D3 of the tightening portion 53 (see FIG. 7) decreases, with the result that the tightening portion 53 may deform when the tightening portion 53 is rotated for tightening by use of a tool such as a plug wrench.
[0007] When the outer diameter of the tightening portion 53 is decreased, while a large wall thickness D3 is maintained, the thickness of the space which is defined between the lower surface of the insulation member 58 and the inner wall surface of the tightening portion 53 and which accommodates the O-ring 59 decreases. In this case, even when the O-ring 59 is pressed by the lower surface of the insulation member 58, the O-ring 59 is not expanded to a sufficient degree, resulting in a failure to establish close contact between the O-ring 59 and the inner wall surface of the tightening portion 53, which in turn results in an impairment in the airtight integrity of the interior of the metallic shell 51.
[0008] A first object of the present invention is to provide a glow plug, the tightening portion of which has increased strength and a reduced across-face dimension.
[0009] A second object of the present invention is to provide a glow plug, the tightening portion of which has increased strength and a reduced across-face dimension, and in which the interior space of a metallic shell has improved airtight integrity.
[0010] To achieve these objects, the present invention provides a glow plug comprising a metallic shell, an insulation member, a center shaft, and a heating element. The metallic shell has a cylindrical male-threaded portion and a tightening portion. The male-threaded portion has a through-hole formed along a center axis and a male thread formed on an outer circumferential surface and adapted to be screw-engaged with a female-threaded hole provided in a diesel engine. The tightening portion is formed to have a through-hole which extends along the center axis to communicate with the through-hole of the male-threaded portion, as well as an outer circumferential surface suitable for engagement with a tool used for screw-engaging the male-threaded portion with the female-threaded hole. The insulation member is fitted into the through-hole of the tightening portion and having a through-hole which extends along the center axis to communicate with the through-hole of the tightening portion. The center shaft is inserted into the metallic shell such that the center shaft passes through the through-hole of the insulation member, the through-hole of the tightening portion, and the through-hole of the male-threaded portion. The heating element is disposed within the metallic shell and is connected to an end of the center shaft located within the metallic shell. The heating element generates heat upon application of electricity to the center shaft. The outer circumferential surface of the tightening portion has a shape corresponding to that of a 12-point nut.
[0011] In the glow plug of the present invention, since the outer circumferential surface of the tightening portion has a shape corresponding to that of a 12-point nut, the wall thickness of the tightening portion (the minimum dimension between the outer circumferential surface and the through-hole of the tightening portion) can be increased as compared with the case where the tightening portion has an outer circumferential shape corresponding to that of a hexagon nut.
[0012] Therefore, even when the across-face dimension of the tightening portion is reduced, the wall thickness thereof can be made large, so that the tightening portion has a higher strength than in the case where the tightening portion has an outer circumferential shape corresponding to that of a hexagon nut.
[0013] The present invention is advantageously employed when a stepped portion is formed on an inner wall surface of the tightening portion to form a surface extending from the inner wall surface toward the center shaft; and a seal member for securing airtight integrity of the interior space of the metallic shell is disposed in a space defined by the stepped portion, the insulation member, and the outer circumference surface of the center shaft.
[0014] In the case where the glow plug has the above-described structure, when the wall of the tightening portion has an excessively large thickness, the diameter of the space which is defined by the stepped portion, the insulation member, and the outer circumference surface of the center shaft and which accommodates the seal member decreases. In this case, the degree of expansion of the seal member generated upon the lower surface of the insulation member pressing the seal member decreases, so that the seal member fails to establish close contact with the stepped portion, the insulation member, and the outer circumference surface of the center shaft, which in turn results in an impairment in the airtight integrity of the interior of the metallic shell. Although the space for accommodating the seal member can be expanded through an increase in the diameter of the through-hole of the tightening portion, this results in a decrease in the wall thickness of the tightening portion.
[0015] In contrast, in the glow plug according to the present invention, even when the across-face dimension of the tightening portion is decreased and the diameter of the through-hole of the tightening portion is increased, a large wall thickness can be secured, as compared with the case where the tightening portion has an outer circumferential shape corresponding to that of a hexagon nut. Therefore, the space for accommodating the seal member can be expanded, while the strength of the tightening portion is ensured.
[0016] Accordingly, the present invention realizes a glow plug in which the tightening portion has an increased strength and the interior space of the metallic shell has improved airtight integrity.
[0017] When the diameter of a portion of the through-hole of the tightening portion where the insulation member is fitted is represented by d1 and the across-face dimension of the tightening portion is represented by L1, a ratio A = d1/L1 preferably falls within a range of 0.77 to 0.82.
[0018] As described above, when the wall of the tightening portion has an excessively small thickness, the strength of the tightening portion decreases; and when the wall of the tightening portion has an excessively large thickness, the airtight integrity of the interior space of the metallic shell decreases. However, as will be described in the embodiment section, when the tightening portion is formed such that the ratio A falls within the above described range, there can be realized a glow plug having improved strength and airtight integrity.
[0019] Preferably, the normal diameter d2 of the male-threaded portion is substantially equal to the across-face dimension L1 of the tightening portion. In this case, the glow plug becomes thin as a whole. The normal diameter d2 is preferably 8 mm or 6 mm.
[0020] Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which:
FIG. 1 is a longitudinal cross section of a glow plug according to an embodiment of the present invention;
FIG. 2A is a view showing an appearance of the rear end of the glow plug shown in FIG. 1;
FIG. 2B is a plan view of a tightening portion of the glow plug as viewed from the rear-end side;
FIG. 3 is a view used for comparison between the tightening portion of the glow plug shown in FIG. 2A and the tightening portion of a conventional glow shown in FIG. 7;
FIG. 4 is a schematic view of a test device used in an experiment performed in relation to the embodiment;
FIG. 5 is a table showing the results of an experiment;
FIG. 6 shows a longitudinal cross section of a conventional glow plug; and
FIG. 7 is a plane view of the tightening portion of the glow plug shown in FIG. 6.
[0021] In the present embodiment, a metal glow plug (sheath glow plug) will be described as a representative example of a glow plug according to the present invention. The end of a glow plug located on the upper side of FIG. 1 will be referred to as the "rear end (voltage application side) of the glow plug," and the other end of the glow plug located on the lower side will be referred to as the "tip end (heat generation side) of the glow plug."
[0022] FIG. 1 is a longitudinal cross section of a glow plug according to the present invention. FIG. 2A is a view showing an appearance of the rear end of the glow plug shown in FIG. 1; and FIG. 2B is a plan view of a tightening portion of the glow plug as viewed from the rear-end side. FIG. 3 is a view used for comparison between the tightening portion of the glow plug shown in FIG. 2A and the tightening portion of a conventional glow shown in FIG. 7.
[0023] As shown in FIG. 1, the glow plug 10 according to the present embodiment comprises a metallic shell 20 and an center shaft 11, which is inserted into the metallic shell 20 in an insulated state. The metallic shell 20 has a male-threaded portion 21 and a tightening portion 22, which is formed integrally with the male-threaded portion 21 to be located on the rear side of the male-threaded portion 21. The male-threaded portion 21 has a tubular shape and has a through-hole 21b, which extends along a center axis 11b and into which the center shaft 11 is inserted. A male thread 21a is formed on an outer circumferential surface of the male-threaded portion 21. The tightening portion 22 has a through-hole 22a, which extends along the center axis 11b and into which the center shaft 11 is inserted. The through-hole 22a communicates with the through-hole 21b of the male-threaded portion 21 and has a diameter larger than that of the through-hole 21b.
[0024] As shown in FIG. 2, the outer circumferential surface of the tightening portion 22 has a shape corresponding to that of a so-called 12-point nut having twelve corner portions of equal angles (i.e. in the shape of a dodecagon). Further, as shown in FIG. 1, a stepped portion 22b is formed at a tip-side end of the through-hole 22a of the tightening portion 22 to form a surface that extends from the inner wall surface of the through-hole 22a toward the center shaft 11 and tapers toward the tip end of the glow plug 10.
[0025] An insulation member 12 is fitted onto the center shaft 11 from its rear end, and the insulation member 12 is fitted into the through-hole 22a of the tightening portion 22. Specifically, the insulation member 12 has a projection that projects toward the tip side and has a shape corresponding to that of the through hole 22a. The projection of the insulation member 12 is fitted into the through-hole 22a of the tightening portion 22.
[0026] The insulation member 12 interposed between the center shaft 11 and the tightening portion 22 insulates the center shaft 11 from the tightening portion 22.
[0027] The insulation member 12 has a through-hole 12a, which extends along the center axis 11b and into which the center shaft 11 is inserted. The through-hole 12a communicates with the through-hole 22a of the tightening portion 22 and has a diameter smaller than that of the through-hole 21b of the male-threaded portion 21. Thus, an interior space 15 is defined between the center shaft 11 and a portion of the metallic shell 20 extending from the stepped portion 22b of the tightening portion 22 to the tip-side end of the male-threaded portion 21. The interior space 15 insulates the center shaft 11 from the above-described portion.
[0028] An O-ring 13 made of rubber is disposed within a space defined by the tip-side end surface of the insulation member 12, the stepped portion 22b of the tightening portion 22, and the center shaft 11. A male thread 11a is formed on the outer circumferential surface of a rear end portion of the center shaft 11, and a round nut 14 is fitted onto the male thread 11a. When the round nut 14 is rotated in a tightening direction, the insulation member 12 is moved toward the tip-end side, so that the O-ring 13 is compressed and expanded radially by the tip-side surface of the insulation member 12.
[0029] Thus, the compressed O-ring 13 establishes close contact with the tip-side surface of the insulation member 12, the stepped portion 22b of the tightening portion 22, and the center shaft 11, whereby the airtight integrity of the interior space of the metallic shell 20 is maintained.
[0030] A rear end portion of a metallic tube 16 is inserted into the tip end portion of the metallic shell 20 such that the tube 16 is exposed to a combustion chamber of the diesel engine. The tip end of the center shaft 11 is inserted into the tube 16. Inside the tube 16 is disposed a heating coil 17, which extends from a rear-side end to a tip-side end of a space formed between the tube 16 and the center shaft 11. The rear end of the heating coil 17 is welded to the tip end of the center shaft 11, and the tip end of the heating coil 17 is welded to the inner wall surface of the tube 16 at the tip end thereof. Further, the interior of the tube 16 is filled with insulating powder 18. The tube 16 is welded to the inner wall surface of the metallic shell 20. Further, at the rear end of the tube 16, a rubber packing 19 is fixedly disposed such that the packing 19 is in close contact with the rear end surface of the tube 16, the inner wall surface of the metallic shell 20, and the outer circumferential surface of the center shaft 11. The packing 19 provides insulation between the interior space of the tube 16 and the interior space of the metallic shell 20 to thereby maintain the airtight integrity of the interior space of the metallic shell 20.
[0031] The tightening portion 22 of the glow plug 10 according to the present embodiment will now be compared with the tightening portion 53 of the conventional glow plug, with reference to FIG. 3.
[0032] As shown in FIG. 3, the diameter d1 of the tightening portion 22 is equal to the diameter d3 of the tightening portion 53, and the across-face dimension L1 of the tightening portion 22 is equal to the across-face dimension L3 of the tightening portion 53. As is apparent from FIG. 3, the wall thickness D1 of the tightening portion 22 of the glow plug 10 according to the present embodiment is greater than the wall thickness D3 of the tightening portion 53 of the conventional glow plug.
[0033] Therefore, the tightening portion 22 of the glow plug 10 according to the present embodiment can have a greater wall thickness as compared with the tightening portion 53 of the conventional glow plug. When the across-face dimension L1 of the tightening portion 22 is equal to the across-face dimension L3 of the tightening portion 53 and the wall thickness D1 of the tightening portion 22 is equal to the wall thickness D3 of the tightening portion 53, the tightening portion 22 of the glow plug 10 according to the present embodiment can have a greater diameter at a portion into which the insulation member is fitted, as compared with the tightening portion 53 of the conventional glow plug.
[0034] In other words, the glow plug according to the present embodiment provides a larger degree of freedom in designing the wall thickness and inner diameter of the tightening portion as compared with the conventional glow plug.
[0035] In the present embodiment, the inner diameter d1 of the tightening portion 22 is determined to satisfy the relationship 6.16 mm ≤ d1 ≤ 6.56 mm. The across-face dimension L1 of the tightening portion 22 is 8 mm, which is the same as the normal diameter d2 of the male thread 21a.
[0036] The glow plug 10 having the above-described structure is attached to a diesel engine as follows. A tool such as a wrench is engaged with the outer circumferential surface of the tightening portion 22, and turns the tool in order to rotate the male-threaded portion 21. As a result, the male-threaded portion 21 is screw-engaged with a female-threaded hole (not shown) formed in the head of the diesel engine. Thus, the glow plug is attached to the diesel engine. When a voltage is applied to the center shaft 11, the heating coil 17 generates heat, so that the tube 16 is heated to a high temperature, whereby fuel within the combustion chamber is ignited.
[0037] Next, an experiment performed by the present inventors will be described with reference to FIGS. 4 and 5.
[0038] FIG. 4 is a view schematically showing a test device used in the experiment. FIG. 5 is a table showing the results of the experiment.
[0039] The present inventors investigated, through an experiment, the effect of the inner diameter d1 and across-face dimension L1 of the tightening portion 22 on the strength of the tightening portion 22 and the airtight integrity provided by the O-ring 13. Specifically, as shown in FIG. 5, the across-face dimension L1 was set to 8 mm, and the strength of the tightening portion 22 and the airtight integrity were measured, while the ratio A (= d1/L1) was varied within the range of 0.74 to 0.84.
[0040] FIG. 4 shows a testing device. The testing device has a base 30 in which is formed a female-threaded hole 30a. The glow plug 10 shown in FIG. 1 is attached to the base 30 via an attachment member 31. Specifically, the attachment 31 has a downwardly-projecting projection having a male thread 31a, which is screw-engaged with the female-threaded hole 30a of the base 30. A space 31b for accommodating the glow plug 10 is formed in the attachment member 31, and a female thread 31c is formed on the upper portion of the wall that defines the space 31b. The male-threaded portion 21 of the glow plug 10 is screw-engaged with the female thread 31c. An injection hole 16a for injecting high-pressure air is formed in the tube 16, and the rubber packing 19 is partially removed in order to introduce the high-pressure air into the space between the packing 19 and the O-ring 13.
[0041] The details of the experiment will be described. The strength of the tightening portion 22 was determined on the basis of whether or not the tightening portion 22 deformed when the tightening portion 22 was tightened by use of an impact wrench. The tightening force of the impact wrench was 15N·m (rotational speed under no load: 4,000 rpm, and pressing force: 0.49 MPa (5 kg/cm2)).
[0042] The results are shown in FIG. 5. When the ratio A (d1/L1) fell within the range of 0.74 to 0.82, the tightening portion 22 did not deform (O). When the ratio A reached 0.84, the tightening portion 22 deformed (X).
[0043] The airtight integrity provided by the O-ring 13 was determined on the basis of whether or not air leaked through the O-ring 13 when high-pressure air was injected into the interior space of the metallic shell 20. The results are shown in FIG. 5. When the ratio A was 0.74, air leakage occurred upon injection of high-pressure air of 1.8 MPa, and when the ratio A was 0.76, air leakage occurred upon injection of high-pressure air of 2.0 MPa.
[0044] However, when the ratio fell within the range of 0.77 to 0.84, air leakage did not occur even upon injection of high-pressure air of 2.5 MPa.
[0045] The results of the above-described experiment demonstrate that when the ratio A is set to fall within the range 0.77 to 0.82, the tightening portion 22 does not deform even when the glow plug is tightened through rotation of the tightening portion 22, and no air leakage occurs.
[0046] When the across-face dimension L1 is set to 8 mm, and the ratio A is changed within the range of 0.77 to 0.82, the inner diameter d1 of the tightening portion 22 changes within the range of 6.16 to 6.56.
[0047] In the above-described glow plug according to the present invention, since the outer circumferential surface of the tightening portion 22 has a shape corresponding to that of a 12-point nut, it becomes possible to increase the strength of the tightening portion 22, while decreasing the across-face dimension of the tightening portion 22. Further, when the ratio A is set to fall within the range of 0.77 to 0.82, it becomes possible to provide a glow plug in which the airtight integrity of the interior space is improved, and the tightening portion has a sufficient strength and a reduced outer diameter.
[0048] Even when the normal diameter d2 of the male-threaded portion 21 is set to 6 mm and the across-face dimension L1 of the tightening portion 22 is set equal to the normal diameter d2, the grow plug can provide the same advantageous effects as those obtained in the case where the normal diameter d2 of the male-threaded portion 21 is set to 8 mm.
1. A glow plug comprising:
a metallic shell (20) having a cylindrical male-threaded portion (21) and a tightening portion (22), said male-threaded portion (21) having a through-hole (21b) formed along a center axis (11b) and a male thread (21a) formed on an outer circumferential surface and adapted to be screw-engaged with a female-threaded hole provided in a diesel engine, and said tightening portion (22) being formed to have a through-hole (22a) which extends along the center axis (11b) to communicate with the through-hole (21b) of said male-threaded portion (21), as well as an outer circumferential surface suitable for engagement with a tool used for screw-engaging said male-threaded portion with said female-threaded hole;
an insulation member (12) fitted into the through-hole (22a) of said tightening portion (22) and having a through-hole (12a) which extends along the center axis (11b) to communicate with the through-hole (22a) of said tightening portion (22);
a center shaft (11) inserted into said metallic shell (20) such that said center shaft (11) passes through the through-hole (12a) of said insulation member (12), the through-hole (22a) of said tightening portion (22), and the through-hole (21b) of said male-threaded portion (21); and
a heating element (17) disposed within said metallic shell (20) and connected to an end of said center shaft (11) located within said metallic shell (20), said heating element (17) generating heat upon application of electricity to said center shaft (11), characterized in that
the outer circumferential surface of said tightening portion (22) has a shape corresponding to that of a 12-point nut.
2. A glow plug according to Claim 1, characterized in that a stepped portion (22b) is
formed on an inner wall surface of said tightening portion (22) to form a surface
extending from the inner wall surface toward said center shaft (11); and a seal member
(13) for securing airtight integrity of the interior space of said metallic shell
(20) is disposed in a space defined by the stepped portion (22b), said insulation
member (12), and the outer circumference surface of said center shaft (11).
3. A glow plug according to Claim 1 or 2, characterized in that when the diameter of
a portion of the through-hole (22a) of said tightening portion (22) where said insulation
member (12) is fitted is represented by d1 and the across-face dimension of said tightening
portion (22) is represented by L1, a ratio A = d1/L1 is in the range of from 0.77
to 0.82.
4. A glow plug according to any one of Claims 1 to 3, characterized in that the diameter
d2 of said male-threaded portion (21) is equal to the across-face dimension L1 of
said tightening portion (22).
5. A glow plug according to any one of Claims 1 to 4, characterized in that the diameter
d2 of said male-threaded portion (21) is 8 mm.
6. A glow plug according to any one of Claims 1 to 4, characterized in that the diameter
d2 of said male-threaded portion (21) is 6 mm.