GLOW PLUG

Description

[0001] The present invention relates to a glow plug, and particularly to a glow plug enabling improvement in rapid temperature rising characteristic.

[0002] Conventionally, a glow plug has been known that has a metallic tube in which a coil is disposed and in which insulating powder is sealed. The glow plug is used as an auxiliary heat source for an internal combustion engine such as a diesel engine of a compression ignition type. Japanese Patent Application Laid-Open (kokai) No. H04-12489 discloses a technique for increasing the filling density of insulating powder sealed in a tube, by reducing the diameter of the tube through performing drawing on the tube.

[0003] EP-A1-3208539 discloses a glow plug on which the precharacterizing portion of claim 1 is based.

[0004] Generally, however, in the above-described conventional technique, as the drawing ratio (the diameter of the tube before the processing/the diameter of the tube after the processing) is increased in order to increase the filling density of the insulating powder, the insulating powder is pulverized into finer particles, and the number of the particles is increased. As a result, the number of contact points between particles through which heat passes within a period in which the heat is conducted from a coil through the particles to the tube, is increased, and thus, heat becomes less likely to be transmitted from the coil to the tube. Therefore, a characteristic in which the temperature rises to a predetermined temperature within a short time period (hereinafter, referred to as "rapid temperature rising characteristic") may be deteriorated, the characteristic being required of the glow plug in order to improve the startability of the internal combustion engine.

[0005] The present invention has been conceived of in order to solve the above-described problem, and an object of the present invention is to provide a glow plug enabling improvement in rapid temperature rising characteristic.

[0006] The present invention provides a glow plug as defined by claim 1.

[0007] In the glow plug of the invention, the relationship is established where the median diameter of the particle diameters of the insulating powder disposed around the front coil and disposed within the first region is larger than the median diameter of the particle diameters of the insulating powder disposed around the rear coil and disposed within the second region, the first region being a part in the direction of the axial line of the tube and including the maximally heat-generating portion at which the temperature of the surface of the tube reaches the maximum temperature, the second region being a part in the direction of the axial line of the tube. Accordingly, the number of contact points between particles through which heat passes within a period in which the heat is conducted from the front coil through the insulating powder disposed within the first region to the first region of the tube becomes relatively less, whereby thermal conductivity from the front coil to the first region of the tube can be improved. On the other hand, the number of contact points between particles of the insulating powder disposed within the second region becomes relatively more, whereby it is possible to suppress the thermal conductivity from the front coil through the insulating powder disposed within the second region to the rear side, of the tube, which includes the second region. That is, heat generated at the front coil can be made less likely to be transferred to the rear side of the tube, and the heat can be made more likely to be transferred to the first region, of the tube, which includes the maximally heat-generating portion. Therefore, the rapid temperature rising characteristic can be improved.

[0008] Further, in a particle size distribution of the insulating powder disposed within the first region, a cumulative value of frequencies of particle diameters not smaller than 60 µm is not lower than 1.0%, whereby the number of contact points between particles of the insulating powder contributing to thermal conduction within the first region can be reduced. Therefore, in addition to the effects in the first and second aspects, the rapid temperature rising characteristic can be further improved.

[0009] Further, in a particle size distribution of the insulating powder disposed within the second region, frequencies of particle diameters not smaller than 60 µm are 0%, whereby the number of contact points between particles of the insulating powder contributing to thermal conduction within the second region can be increased. Therefore, heat can be made less likely to be transferred to the rear side of the tube, and thus, in addition to the effect in the third aspect, the rapid temperature rising characteristic can be further improved.

[0010] In the glow plug according to an embodiment, in the particle size distribution of the insulating powder disposed within the first region, a cumulative value of frequencies of particle diameters not smaller than 80 µm is not lower than 0.1%, whereby the number of contact points between particles of the insulating powder contributing to thermal conduction within the first region can be reduced. Therefore, in addition to the effect in any of the first to fourth aspects, the rapid temperature rising characteristic can be further improved.

[0011] In the glow plug according to an embodiment, in the particle size distribution of the insulating powder disposed within the second region, frequencies of particle diameters not smaller than 80 µm are 0%, whereby the number of contact points between particles of the insulating powder contributing to thermal conduction within the second region can be increased. Therefore, heat can be made less likely to be transferred to the rear side of the tube, and thus, in addition to the effect in the fifth aspect, the rapid temperature rising characteristic can be further improved.

[0012] The invention will be further described by way of examples with reference to the accompanying drawings, in which:

FIG. 1 is a half sectional view of a glow plug according to a first embodiment of the present invention.

FIG. 2 is a partially enlarged sectional view of the glow plug.

FIG. 3 is an example of a particle size distribution of insulating powder disposed within a first region.

FIG. 4 is an example of a particle size distribution of the insulating powder disposed within a second region.

FIG. 5 is a partially enlarged sectional view of a glow plug according to a second embodiment.

[0013] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. A glow plug 10 according to a first embodiment of the present invention will be described with reference to FIG. 1 and FIG. 2. FIG. 1 is a half sectional view of the glow plug 10, and FIG. 2 is a partially enlarged sectional view of the glow plug 10. In FIG. 1 and FIG. 2, the lower side of the drawing sheet is referred to as a front side of the glow plug 10, and the upper side of the drawing sheet is referred to as a rear side of the glow plug 10 (the same applies to FIG. 5).

[0014] As shown in FIG. 1, the glow plug 10 includes a center shaft 20, a metal shell 30, a tube 40, and a coil 50. These members are assembled along an axial line O of the glow plug 10. The glow plug 10 is an auxiliary heat source used at the time of, for example, starting of an internal combustion engine (not shown) exemplified by a diesel engine.

[0015] The center shaft 20 is a columnar metallic conductor, and is a member for supplying power to the coil 50. The center shaft 20 has a front end to which the coil 50 is electrically connected. The center shaft 20 is inserted in the metal shell 30 in a state where a rear end thereof projects from the metal shell 30.

[0016] In the present embodiment, the center shaft 20 has a rear end portion on which a connection portion 21 that is an external thread is formed. The center shaft 20 has the rear end portion on which an O-ring 22 made from insulating rubber, an insulator 23 that is a tubular member made from synthetic resin, a ring 24 that is a metallic tubular member, and a metallic nut 25 are assembled in order from the front side. The connection portion 21 is a portion to which a connector (not shown) of a cable for supplying power from a power supply such as a battery is to be connected. The nut 25 is a member for fixing the connected connector (not shown).

[0017] The metal shell 30 is a substantially cylindrical member formed from carbon steel or the like. The metal shell 30 has an axial hole 31 penetrating therethrough along the axial line O, and has an outer circumferential surface on which a screw portion 32 is formed. A tool engagement portion 33 is formed on the rear side of the metal shell 30 relative to the screw portion 32. The axial hole 31 is a through hole in which the center shaft 20 is inserted. The inner diameter of the axial hole 31 is larger than the outer diameter of the center shaft 20, and thus, a gap is formed between the center shaft 20 and the axial hole 31. The screw portion 32 is an external thread to be fitted to the internal combustion engine (not shown). The tool engagement portion 33 is a portion having such a shape (e.g., hexagon) as to be engaged with a tool (not shown) used for fitting and removing the screw portion 32 into and from a screw hole (not shown) of the internal combustion engine.

[0018] The metal shell 30 holds the center shaft 20 at the rear side of the axial hole 31 via the O-ring 22 and the insulator 23. By the ring 24 being crimped to the center shaft 20 in a state where the ring 24 is in contact with the insulator 23, the position in the axial direction of the insulator 23 is fixed. The rear side of the metal shell 30 and the ring 24 are insulated from each other by the insulator 23. In the metal shell 30, the tube 40 is fixed to the front side of the axial hole 31.

[0019] The tube 40 is a metallic tubular body having a closed front end. Examples of a material of the tube 40 include heat resistant alloys such as a nickel-based alloy and stainless steel. The tube 40 has: a small-diameter portion 42 having a front end closed by a front end portion 41; a transitional portion 43 disposed adjacent to the rear end of the small-diameter portion 42 and having a diameter increasing toward the rear side; and a large-diameter portion 44 disposed adjacent to the rear end of the transitional portion 43 and having an outer diameter larger than the outer diameter of the small-diameter portion 42. These portions 42, 43, and 44 are contiguous to each other in a direction of the axial line O. In the present embodiment, the outer diameter of each of the small-diameter portion 42 and the large-diameter portion 44 is uniform over the entire length thereof in the direction of the axial line O. The transitional portion 43 is formed in a tapered shape so as to connect the small-diameter portion 42 and the large-diameter portion 44 which have different outer diameters. The tube 40 is fixed to the metal shell 30 by the large-diameter portion 44 being press-fitted into the axial hole 31.

[0020] In the large-diameter portion 44, the front side of the center shaft 20 is inserted. The inner diameter of the large-diameter portion 44 is larger than the outer diameter of the center shaft 20, and thus, a gap is formed between the center shaft 20 and the large-diameter portion 44. A seal member 26 is a cylindrical insulating member sandwiched between the front side of the center shaft 20 and the large-diameter portion 44. The seal member 26 maintains the gap between the center shaft 20 and the tube 40, and seals a portion between the center shaft 20 and the tube 40. The coil 50 is housed along the axial line O in the tube 40. Insulating powder 60 is sealed in the tube 40 by the seal member 26.

[0021] As shown in FIG. 2, a part of the helically formed coil 50 is disposed in the small-diameter portion 42. The coil 50 is composed of a front coil 51 and a rear coil 52. The front coil 51 is disposed in the small-diameter portion 42. In the present embodiment, the front coil 51 is made from a high-melting-point metal containing W or Mo as a main component. The front coil 51 may be made from one of these elements alone or an alloy containing any of these elements as a main component. The phrase "W or Mo as a main component" means that the total content of W or Mo relative to the entire content of the material of the coil is not lower than 50 wt%.

[0022] The front coil 51 has a front end joined to the front end portion 41 of the tube 40 by welding. The rear coil 52 is joined to the rear end of the front coil 51 by welding. In the present embodiment, the rear coil 52 is formed in a shape of a helix having a pitch wider than the pitch of the front coil 51. Between the front coil 51 and the rear coil 52, a fusion portion 53 obtained by solidification of welded metal having been melted by welding is present. The rear coil 52 is connected in series to the front coil 51 via the fusion portion 53.

[0023]  In the present embodiment, the rear coil 52 is formed from a conductive material having a resistance ratio R2 lower than a resistance ratio R1 of the front coil 51. The resistance ratio refers to a "ratio of a resistance value at 1000°C to a resistance value at 20°C", and, the larger the value of the resistance ratio is, the larger the resistance value at high temperature becomes. Examples of a material of the rear coil 52 include a FeCr alloy and a NiCr alloy. The rear coil 52 is housed along the axial line O from the small-diameter portion 42 of the tube 40 to the large-diameter portion 44 thereof, and has a rear end joined to the center shaft 20. The center shaft 20 is electrically connected to the tube 40 via the rear coil 52 and the front coil 51.

[0024] A resistance value R₂ at 20°C of the rear coil 52 is set to a value larger than a resistance value R₁ at 20°C of the front coil 51. Therefore, current I (rush current) is ensured that flows to the front coil 51, at a normal temperature. When a voltage V is applied between the center shaft 20 and the tube 40, the current I obtained as a result of dividing the voltage V by a sum R₁+R₂ of the resistance value R₁ of the front coil 51 and the resistance value R₂ of the rear coil 52 flows to the front coil 51 and the rear coil 52. The heat generation amount, per unit time, of the front coil 51 is R₁·I², and the heat generation amount, per unit time, of the rear coil 52 is R₂·I².

[0025] The rear coil 52 has the resistance ratio R2 lower than the resistance ratio R1 of the front coil 51, and thus, in association with increase in the temperature due to heat generation by the front coil 51 and the rear coil 52, the resistance value R₁ of the front coil 51 becomes larger than the resistance value R₂ of the rear coil 52. As a result, the heat generation amount R₁·I², per unit time, of the front coil 51 can be made larger than the heat generation amount R₂·I², per unit time, of the rear coil 52. Since the front coil 51 is formed from a high-melting-point metal containing W or Mo as a main component, the heat generation temperature can be increased. Accordingly, the temperature of the front coil 51 can be increased to a desired temperature (e.g., 1000°C), and the temperature of the surface of the small-diameter portion 42 can be increased to the desired temperature by thermal conduction through the insulating powder 60.

[0026]  The insulating powder 60 is a powder having electrical insulation property and having thermal conductivity at high temperature. The insulating powder 60 is filled at: a portion between the coil 50 (front coil 51 and rear coil 52) and the tube 40; a portion between the center shaft 20 and the tube 40; and the inner side of the coil 50. The insulating powder 60 has: a function to transfer heat from the coil 50 to the tube 40; a function to prevent short-circuiting between the coil 50 and the tube 40; and a function to prevent wire breakage by causing the coil 50 to be less likely to vibrate.

[0027] For the insulating powder 60, one type or multiple types of oxide powders such as MgO, CaO, Al₂O₃, ZrO₂, and SiO₂ are used, for example. A powder such as Si may be added to the oxide powders. The insulating powder 60 preferably contains MgO since MgO can maintain a desired coefficient of thermal conductivity among the oxide powders. With respect to the overall mass of the insulating powder 60, the insulating powder 60 preferably contains not lower than 80 mass% and not higher than 100 mass% of MgO, and more preferably contains not lower than 99 mass% and lower than 100 mass% of MgO.

[0028] The front coil 51 has such an irregular pitch that a pitch on the front side is set to be narrower than a pitch on the rear side, and thus, a maximally heat-generating portion 45 at which the temperature of the surface of the small-diameter portion 42 reaches the maximum temperature, is formed on the front side of the small-diameter portion 42 (an area surrounding the narrower-pitch portion of the front coil 51). In the glow plug 10, by taking into consideration the thermal conductivity of the insulating powder 60, a particular relationship is established between: a particle size distribution of the insulating powder 60 disposed within a first region 46 which includes the maximally heat-generating portion 45 and which is a part in the direction of the axial line O of the small-diameter portion 42; and a particle size distribution of the insulating powder 60 disposed within a second region 47 which is a part in the direction of the axial line O of the large-diameter portion 44. The details thereof will be described later.

[0029] The maximally heat-generating portion 45 can be identified by measuring, with use of a radiation thermometer or a thermocouple, the temperature of the surface of the small-diameter portion 42 while the electrically energized coil 50 is generating heat. The first region 46 is a region that includes the maximally heat-generating portion 45 and that is a part in the direction of the axial line O of the small-diameter portion 42. In the present embodiment, the first region 46 is a tubular region, of the small-diameter portion 42, that surrounds the coil 50. The second region 47 is a region that is a part in the direction of the axial line O of the large-diameter portion 44. In the present embodiment, the second region 47 is a tubular region, of the large-diameter portion 44, that surrounds the rear coil 52.

[0030] The glow plug 10 is manufactured as follows, for example. First, resistive heat generation wires each having a predetermined composition are processed into coil shapes, thereby producing the front coil 51 and the rear coil 52. Then, ends of the front coil 51 and the rear coil 52 are joined to each other by welding thereby to form the coil 50, and the coil 50 is joined to the front end of the center shaft 20.

[0031] Meanwhile, a metal steel tube (element tube) having a predetermined composition is formed so as to have a diameter larger than a final dimension for the tube 40, and the diameter of the front end thereof is made smaller than that of the other portion, thereby producing a tube precursor having an open front end in a front-tapered shape. The coil 50 integrated with the center shaft 20 is inserted into the tube precursor, and the front end of the coil 50 is disposed at a front-tapered opening portion of the tube precursor. The opening portion of the tube precursor and the coil 50 are welded to each other, a front end portion of the tube precursor is closed thereby to form the front end portion 41, and a front end portion of the coil 50 is embedded in the front end portion 41, thereby forming a heater precursor in which the coil 50 (front coil 51 and rear coil 52) is housed in the tube 40 (element tube).

[0032] Then, after the insulating powder 60 is filled in the tube 40 of the heater precursor, the seal member 26 is inserted between the center shaft 20 and an opening portion of the rear end of the tube 40, thereby sealing the tube 40. Next, swaging is performed on the tube 40 until the tube 40 has a predetermined outer diameter. By swaging being performed on the tube 40 (element tube) to narrow the tube 40, the filling density of the insulating powder 60 can be increased and unevenness in filling can be reduced.

[0033] Thereafter, the tube 40 after the swaging is press-fitted and fixed into the axial hole 31 of the metal shell 30, and, from the rear end of the center shaft 20, the O-ring 22 and the insulator 23 are fitted between the metal shell 30 and the center shaft 20. The center shaft 20 is crimped with use of the ring 24, thereby obtaining the glow plug 10.

[0034] Next, a particle size distribution of the insulating powder 60 sealed in the tube 40 will be described with reference to FIG. 3 and FIG. 4. FIG. 3 is an example of a particle size distribution of the insulating powder 60 disposed within the first region 46 of the tube 40, and FIG. 4 is an example of a particle size distribution of the insulating powder 60 disposed within the second region 47 of the tube 40. The particle size distributions described below are volume-based particle size distributions measured by a laser diffraction method. Each of FIG. 3 and FIG. 4 schematically shows a frequency distribution and a cumulative distribution (minus sieve) of particle diameters.

[0035] By using a laser diffraction type particle size distribution measurement device (e.g., HORIBA LA-750 manufactured by HORIBA, Ltd.), the particle size distribution of the insulating powder 60 can be measured as follows. Before destructing the glow plug 10 and taking out the insulating powder 60 from the tube 40, the glow plug 10 is electrically energized to identify the position of the maximally heat-generating portion 45. In a case where a plurality of glow plugs 10 having the same configuration are available, the insulating powder 60 may be taken out from a glow plug different from a glow plug of which the position of the maximally heat-generating portion 45 is identified.

[0036] The insulating powder 60 disposed on the inner side of the first region 46 including the maximally heat-generating portion 45 is taken out from the glow plug 10, thereby preparing a specimen. In the present embodiment, first, the tube 40 is cut along a plane that is orthogonal to the axial line O and that includes an area at and around the fusion portion 53. The position of the fusion portion 53 can be identified with use of an X-ray nondestructive inspection device. Then, the coil 50 disposed in the tube 40 so as to be present on the front end portion 41 side is led out from the tube 40, and impact is applied to the coil 50, thereby taking out particles filled on the inner side of the coil 50. Similarly, impact is applied to the tube 40, thereby taking out the particles in the tube 40.

[0037] Next, the insulating powder 60 disposed on the inner side of the second region 47 is taken out from the glow plug 10, thereby preparing a specimen. In the present embodiment, first, the tube 40 is cut along a plane that is orthogonal to the axial line O and that includes the boundary between the large-diameter portion 44 and the transitional portion 43 and along a plane that is orthogonal to the axial line O and that includes the front end of the center shaft 20. The position of the front end of the center shaft 20 can be identified with use of the X-ray nondestructive inspection device. After the tube 40 is cut, the coil 50 in the tube 40 is led out from the tube 40, and impact is applied to the coil 50, thereby taking out particles filled on the inner side of the coil 50. Similarly, impact is applied to the tube 40, thereby taking out particles in the tube 40.

[0038] The particles having been taken out are in a state of being condensed so as to have a conglomerate form, and thus, are ground with use of a mortar, thereby pulverizing the conglomerate. It has been known that, since the particles are hard, the particles (primary particles) are not pulverized even by grinding the conglomerate with use of the mortar and a muller held by a hand, resulting in no influence on a measurement result. While the particles having been ground with use of the mortar are observed with use of a magnifier, impurities are removed. In this manner, not less than 0.35 g of specimen is prepared for each time of measurement.

[0039] Then, the prepared specimen (e.g., in two to four spatulas) is dispersed in a dispersion medium (e.g., 150 cc of a solution containing 0.2 mass% of sodium hexametaphosphate). Examples of a dispersion method for the specimen include a method in which the resultant solution is stirred for 3 minutes with use of an external homogenizer, and thereafter, the stirred solution is further stirred for 2 minutes with use of an ultrasonic probe incorporated in the laser diffraction type particle size distribution measurement device. With use of the laser diffraction type particle size distribution measurement device, a particle size distribution of the specimen dispersed in the dispersion medium is measured. The measurement of the particle size distribution is performed three times, and an average value based on the three times of measurement is obtained.

[0040] The particles of the insulating powder 60 sometimes exist as primary particles, or sometimes exist as secondary particles. Although the insulating powder 60 may exist in the form of either the primary particles or the secondary particles, the insulating powder 60 preferably exists as the primary particles. In a case where the particles exist as the secondary particles, a large number of spaces exist in the secondary particles, and thus, the spaces may serve as heat insulating layers (barriers), resulting in reduction in the thermal conductivity of the insulating powder 60. Normally, MgO does not form the secondary particles, but exists as the primary particles. Therefore, the particles composing the insulating powder 60 are preferably particles containing MgO as a main component.

[0041] As shown in FIG. 3 and FIG. 4, a median diameter (a particle diameter at which the cumulative value of frequencies is 50%; see FIG. 3) of particle diameters of the insulating powder 60 disposed within the first region 46 (see FIG. 2) is larger than a median diameter (see FIG. 4) of particle diameters of the insulating powder 60 disposed within the second region 47 (see FIG. 2). Specifically, the median diameter of the particle diameters of the insulating powder 60 disposed within the first region 46 is 22.0 µm, and the median diameter of the particle diameters of the insulating powder 60 disposed within the second region 47 is 9.5 µm.

[0042] The median diameters of particle diameters of the insulating powder 60 disposed within the first region 46 and the second region 47 can be set by adjusting drawing ratios of swaging (the diameter of the tube 40 before the processing/the diameter of the tube 40 after the processing) at the small-diameter portion 42 and the large-diameter portion 44 of the tube 40. Generally, as the drawing ratio is increased, the particles of the insulating powder 60 filled in the tube 40 are more pulverized and the median diameter of the particle diameters becomes smaller. Therefore, by setting the drawing ratio at the small-diameter portion 42 to be lower than the drawing ratio at the large-diameter portion 44, the median diameter of the particle diameters of the insulating powder 60 disposed within the first region 46 (small-diameter portion 42) can be made larger than the median diameter of the particle diameters of the insulating powder 60 disposed within the second region 47 (large-diameter portion 44).

[0043] In the glow plug 10, a relationship is established where the median diameter of the particle diameters of the insulating powder 60 disposed within the first region 46 is larger than the median diameter of the particle diameters of the insulating powder 60 disposed within the second region 47, and thus, the number, per unit volume, of the particles of the insulating powder 60 disposed within the first region 46 can be made less than the number, per unit volume, of the particles of the insulating powder 60 disposed within the second region 47.

[0044] As a result, the number of contact points between particles through which heat passes within a period in which the heat is conducted from the coil 50 through the insulating powder 60 within the first region 46 to the first region 46, can be made relatively less. Accordingly, thermal conductivity from the coil 50 to the small-diameter portion 42 can be improved. On the other hand, the number of contact points between particles through which heat passes when heat is conducted through the insulating powder 60 within the second region 47, can be made relatively more. Accordingly, it is possible to suppress the thermal conductivity from the coil 50 to the large-diameter portion 44. As a result, heat generated at the coil 50 can be made less likely to be transferred to the large-diameter portion 44, and can be made more likely to be transferred to the first region 46 including the maximally heat-generating portion 45 of the small-diameter portion 42. Therefore, the rapid temperature rising characteristic can be improved.

[0045] The phrase "a relationship is established where the median diameter of the particle diameters of the insulating powder 60 disposed within the first region 46 is larger than the median diameter of the particle diameters of the insulating powder 60 disposed within the second region 47" means that the first region 46 and the second region 47 in which the predetermined relationship is established between the median diameter of the particle diameters of the insulating powder 60 disposed in the small-diameter portion 42 and the median diameter of the particle diameters of the insulating powder 60 disposed in the large-diameter portion 44, exist in the small-diameter portion 42 and the large-diameter portion 44, respectively. This is because high thermal conductivity in the small-diameter portion 42 and low thermal conductivity in the large-diameter portion 44 can be obtained if the portions in which such a relationship is established exist in the small-diameter portion 42 and the large-diameter portion 44. Therefore, the above-described predetermined relationship does not have to be established in all of the particle size distributions of the insulating powder 60 extracted from arbitrary positions in the small-diameter portion 42 and the large-diameter portion 44.

[0046] Here, the minimum ranges (the lengths in the direction of the axial line O) of the first region 46 and the second region 47 are each set to such a length that a specimen for measurement of a particle size distribution by the laser diffraction method can be extracted in a required amount. The lengths in the direction of the axial line O of the first region 46 and the second region 47 may be equal or different. The second region 47 is preferably set so as to include a portion, of the large-diameter portion 44, that surrounds the front side relative to the front end of the center shaft 20 (the second region 47 is preferably set at a portion, of the large-diameter portion 44, that surrounds the rear coil 52). This is because heat of the coil 50 can be made less likely to be transmitted to the large-diameter portion 44 at the rear side relative to the front end of the center shaft 20 by the insulating powder 60 if the median diameter of the particle diameters of the insulating powder 60 disposed at the portion, of the large-diameter portion 44, that surrounds the front side relative to the front end of the center shaft 20 is smaller than the median diameter of the particle diameters of the insulating powder 60 disposed within the first region 46.

[0047] As shown in FIG. 3, in the particle size distribution of the insulating powder 60 disposed within the first region 46, the cumulative value of frequencies of particle diameters not smaller than 60 µm is 4.0%. In this manner, the cumulative value of frequencies of particle diameters not smaller than 60 µm is preferably not lower than 1.0%. If particles having particle diameters not smaller than 60 µm are included within the first region 46 at not lower than 1.0%, the number of contact points between particles can be more reduced, whereby the rapid temperature rising characteristic can be improved. In the particle size distribution of the insulating powder 60 disposed within the first region 46, the cumulative value of frequencies of particle diameters not smaller than 60 µm is preferably not higher than 10%. This is because, if the cumulative value of frequencies of particle diameters not smaller than 60 µm exceeds 10%, the amount of particles having small particle diameters and filling the gaps formed by contact between particles having particle diameters not smaller than 60 µm is insufficient and the gaps are less likely to be filled, and thus, the coefficient of thermal conductivity tends to be reduced.

[0048] Furthermore, in this case, in the particle size distribution of the insulating powder 60 disposed within the second region 47 (see FIG. 4), frequencies of particle diameters not smaller than 60 µm are preferably 0%. Accordingly, the number of contact points between particles of the insulating powder 60 disposed within the second region 47 is increased so that the coefficient of thermal conductivity is reduced to suppress heat dissipation within the second region 47, whereby the rapid temperature rising characteristic is improved.

[0049] As shown in FIG. 3, in the particle size distribution of the insulating powder 60 disposed within the first region 46, the cumulative value of frequencies of particle diameters not smaller than 80 µm is 1.5%. In this manner, the cumulative value of frequencies of particle diameters not smaller than 80 µm is preferably not lower than 0.1%. If particles having particle diameters not smaller than 80 µm are included within the first region 46 at not lower than 0.1%, the number of contact points between particles can be more reduced, whereby the rapid temperature rising characteristic can be improved. In the particle size distribution of the insulating powder 60 disposed within the first region 46, the cumulative value of frequencies of particle diameters not smaller than 80 µm is preferably not higher than 5%. This is because, if the cumulative value of frequencies of particle diameters not smaller than 80 µm exceeds 5%, the amount of particles having small particle diameters and filling the gaps between particles having particle diameters not smaller than 80 µm is insufficient and the gaps are less likely to be filled, and thus, the coefficient of thermal conductivity tends to be reduced.

[0050] Furthermore, in this case, in the particle size distribution of the insulating powder 60 disposed within the second region 47 (see FIG. 4), frequencies of particle diameters not smaller than 80 µm are preferably 0%. Accordingly, the number of contact points between particles of the insulating powder 60 disposed within the second region 47 is increased so that the coefficient of thermal conductivity is reduced to suppress heat dissipation within the second region 47, whereby the rapid temperature rising characteristic is improved.

[0051] Next, a second embodiment will be described with reference to FIG. 5. In the first embodiment, the glow plug 10 including the tube 40 formed so as to have the small-diameter portion 42 and the large-diameter portion 44 having outer diameters different from each other, has been described. On the other hand, in the second embodiment, a glow plug 70 including a tube 71 having an outer diameter that is uniform over the entire length thereof in the direction of the axial line O except at a front end portion 72, will be described. The same components as the components described in the first embodiment are denoted by the same reference numerals, and the description thereof is omitted. FIG. 5 is a partially enlarged sectional view including the axial line O of the glow plug 70 according to the second embodiment. In FIG. 5, the rear side of the glow plug 70 is not shown. The tube 71 of the glow plug 70 is fixed to the front side of the axial hole 31 of the metal shell 30 (see FIG. 1).

[0052] The tube 71 is a metallic tubular body having a front end closed by the front end portion 72. Examples of a material of the tube 71 include heat resistant alloys such as a nickel-based alloy and stainless steel. In the tube 71, the front side of the center shaft 20 is inserted on the rear side. The seal member 26 (see FIG. 1) maintains the gap between the center shaft 20 and the tube 71, and seals a portion between the center shaft 20 and the tube 71. A coil 80 is composed of a front coil 81 and a rear coil 82. The coil 80 is housed along the axial line O in the tube 71. The insulating powder 60 is sealed in the tube 71 by the seal member 26.

[0053] In the present embodiment, the front coil 81 is made from a high-melting-point metal containing W or Mo as a main component. The front coil 81 has a front end joined to the front end portion 72 of the tube 71 by welding. The rear coil 82 is joined to the rear end of the front coil 81 by welding. Between the front coil 81 and the rear coil 82, a fusion portion 83 obtained by solidification of welded metal having been melted by welding is present. The rear coil 82 is connected in series to the front coil 81 via the fusion portion 83. In the present embodiment, the rear coil 82 is formed from a conductive material having a resistance ratio R2 lower than a resistance ratio R1 of the front coil 81. Examples of a material of the rear coil 82 include a FeCr alloy and a NiCr alloy. The rear coil 82 has a rear end joined to the center shaft 20. The center shaft 20 is electrically connected to the tube 71 via the rear coil 82 and the front coil 81.

[0054] The front coil 81 has such an irregular pitch that a pitch on the front side is set to be narrower than a pitch on the rear side, and thus, a maximally heat-generating portion 73 at which the temperature of the surface of the tube 71 reaches the maximum temperature is formed in an area, of the tube 71, that surrounds the narrower-pitch portion of the front coil 81. A first region 74 is a region that includes the maximally heat-generating portion 73 and that is a part in the direction of the axial line O of the tube 71, and is a tubular region, of the tube 71, that surrounds at least a part of the front coil 81. A second region 75 is a region that is a part in the direction of the axial line O of the tube 71, and is a tubular region, of the tube 71, that surrounds at least a part of the rear coil 82.

[0055] In the present embodiment, the first region 74 is set in a range stretching over the entire length of the front coil 81, and the second region 75 is set in a range stretching over the entire length of the rear coil 82. However, the present invention is not necessarily limited thereto. The minimum ranges (the lengths in the direction of the axial line O) of the first region 74 and the second region 75 are each set to such a length that a specimen for measurement of a particle size distribution by the laser diffraction method can be extracted in a required amount.

[0056] Similarly to the first embodiment, in the glow plug 70, a relationship is established where the median diameter of the particle diameters of the insulating powder 60 disposed within the first region 74 and disposed around the front coil 81 (see FIG. 3) is larger than the median diameter of the particle diameters of the insulating powder 60 disposed within the second region 75 and disposed around the rear coil 82 (see FIG. 4). The median diameters of the particle diameters of the insulating powder 60 disposed within the first region 74 and the second region 75 can be set by partially adjusting drawing ratios of swaging (the diameter of the tube 71 before the processing/the diameter of the tube 71 after the processing) at the first region 74 and the second region 75. According to the glow plug 70, the same advantageous effects as those of the glow plug 10 described in the first embodiment can be obtained.

[0057] The phrase "a relationship is established where the median diameter of the particle diameters of the insulating powder 60 disposed within the first region 74 is larger than the median diameter of the particle diameters of the insulating powder 60 disposed within the second region 75" means that the portions in which the predetermined relationship is established between the median diameter of the particle diameters of the insulating powder 60 disposed around the front coil 81 and disposed within the first region 74 and the median diameter of the particle diameters of the insulating powder 60 disposed around the rear coil 82 and disposed within the second region 75, exist in the tube 71. This is because both high thermal conductivity around the front coil 81 and low thermal conductivity around the rear coil 82 can be achieved if the portions in which such a relationship is established exist around the front coil 81 and the rear coil 82. Therefore, the above-described predetermined relationship does not have to be established in all of the particle size distributions of the insulating powder 60 extracted from arbitrary positions in the tube 71.

[Examples]

[0058] The present invention will be described further in detail on the basis of examples, but the present invention is not limited to the examples.

(Production of samples)

[0059] Samples having the same structure as that of the glow plug 10 shown in FIG. 1 were produced. First, the front coils 51 were formed from a wire material having a diameter φ of 0.20 mm and made purely from W. Similarly, the helical rear coils 52 were formed from a wire material having a diameter φ of 0.38 mm and made from a NiCr alloy. Then, the rear coils 52 were joined to the front coils 51 by welding. The rear coils 52 and the front coils 51 joined to each other (coils 50) had a resistance value of 0.31 Ω at 20°C.

[0060] Then, the coils 50 were joined to various metal steel tubes having outer diameters different from each other to form the heater precursors, and the insulating powder 60 was filled in the heater precursors, and thereafter, the resultant heater precursors were subjected to swaging, thereby shaping the tubes 40. In each of the tubes 40 after the swaging, the outer diameter φ of the small-diameter portion 42 was 3.25 mm and the outer diameter φ of the large-diameter portion 44 was 4.0 mm. The metal shells 30 were assembled to the tubes 40, thereby obtaining glow plugs of samples 1 to 18 indicated in Table 1.

[Table 1]

No.	Second region	First region			Rapid temperature rising characteristic
	Median diameter (µm)	Median diameter (µm)	≥60 µm(%)	≥80 µm(%)
1	12	5	0	0	D
2	12	9	0	0	D
3	12	11	0	0	D
4	12	18	0	0	C
5	12	20	0	0	C
6	12	25	0	0	C
7	12	25	1.0	0	B
8	12	25	3.5	0	B
9	12	25	5.4	0	B
10	12	25	6.8	0	B
11	12	25	8.4	0	B
12	12	25	10.0	0	B
13	12	25	5.0	0.1	A
14	12	25	6.5	1.5	A
15	12	25	7.2	2.2	A
16	12	25	8.4	3.4	A
17	12	25	9.2	4.2	A
18	12	25	10.0	5.0	A

[0061] Table 1 indicates: the median diameter of particle diameters of the insulating powder 60 disposed within the second region 47; the median diameter of particle diameters of the insulating powder 60 disposed within the first region 46; the cumulative value of frequencies of particle diameters not smaller than 60 µm of the insulating powder 60 disposed within the first region 46; and the cumulative value of frequencies of particle diameters not smaller than 80 µm of the insulating powder 60 disposed within the first region 46. These are results obtained as follows: after electrical energization tests to evaluate rapid temperature rising characteristic were performed, the tubes 40 were cut and the insulating powders 60 were taken out to prepare the specimens for measurement as described above, and the particle size distributions were measured with use of the laser diffraction type particle size distribution measurement device (HORIBA LA-750).

[0062] In order to set the first region 46, the maximally heat-generating portion 45 at which the temperature of the surface of the tube 40 reaches the maximum temperature needs to be identified. When each sample was electrically energized and the temperature of the surface of the tube 40 was measured with use of the radiation thermometer, the temperature of a portion that is distant by 2 mm from the front end of the tube 40 toward the rear side in the direction of the axial line O was the highest. Thus, this portion was identified as the maximally heat-generating portion 45.

(Electrical energization test)

[0063] A test method by which the rapid temperature rising characteristics of the samples were evaluated will be described. In each test, a DC voltage of 11 V was applied between the connection portion 21 and the metal shell 30 of each of the samples, and the temperature of the maximally heat-generating portion 45 was measured with use of a PR thermocouple joined to the maximally heat-generating portion 45 (a surface, of the tube 40, that was distant by 2 mm from the front end of the tube 40 toward the rear side in the direction of the axial line O). Samples in each of which the temperature of the maximally heat-generating portion 45 obtained 2 seconds after the voltage was applied was not lower than 950°C, is evaluated as "particularly excellent (A)", samples in each of which this temperature was not lower than 900°C and lower than 950°C is evaluated as "excellent (B)", samples in each of which this temperature was not lower than 850°C and lower than 900°C is evaluated as "satisfactory (C)", and samples in each of which this temperature was lower than 850°C is evaluated as "not good (D)". The measurement of this temperature may be performed with use of a radiation thermometer instead of the PR thermocouple.

(Measurement of particle size distribution)

[0064] The tube 40 was cut along a plane that was orthogonal to the axial line O of each sample and that included an area at and around the fusion portion 53, particles in the tube 40 at the front end portion 41 side were taken out, and the particles were used as the insulating powder 60 disposed within the first region 46. In addition, the tube 40 was cut along a plane that was orthogonal to the axial line O of each sample and that included the boundary between the large-diameter portion 44 and the transitional portion 43 and along a plane that was orthogonal to the axial line O of the sample and that included the front end of the center shaft 20, particles in the large-diameter portion 44 cut in a round slice were taken out, and the particles were used as the insulating powder 60 disposed within the second region 47. The particles having been taken out were ground with use of a mortar thereby to pulverize the conglomerate, and the resultant particles were used as the specimen for measurement.

[0065] The specimen was dispersed in 150 cc of a solution containing 0.2 mass% of sodium hexametaphosphate, and the resultant solution was stirred for 3 minutes with use of an external homogenizer, and thereafter, the stirred solution was further stirred for 2 minutes with use of an ultrasonic probe incorporated in a particle size distribution measurement device (HORIBA LA-750). Then, a particle size distribution of the resultant specimen dispersed in the dispersion medium was measured. The measurement of the particle size distribution was performed three times, and an average value of the values obtained by the three times of measurement was obtained. The average value was rounded to the first decimal place.

[0066] As shown in Table 1, in each of samples 1 to 3, the median diameter of the particle diameters of the insulating powder 60 within the first region 46 was smaller than the median diameter of the particle diameters of the insulating powder 60 within the second region 47. In each of samples 4 to 18, the median diameter of the particle diameters of the insulating powder 60 within the first region 46 was larger than the median diameter of the particle diameters of the insulating powder 60 within the second region 47. It has been found that samples 4 to 18 have more excellent rapid temperature rising characteristic than samples 1 to 3. It is assumed that, in each of samples 4 to 18, the insulating powder 60 made it less likely for heat of the coil 50 to be transferred to the rear side and made it more likely for the heat of the coil 50 to be transferred to the first region 46 including the maximally heat-generating portion 45 than in each of samples 1 to 3.

[0067] In each of samples 7 to 18, the cumulative value of frequencies of particle diameters not smaller than 60 µm of the insulating powder 60 within the first region 46 was 1.0 to 10.0%. On the other hand, in each of samples 4 to 6, frequencies of particle diameters not smaller than 60 µm of the insulating powder 60 within the first region 46 were 0%. It has been found that samples 7 to 18 have more excellent rapid temperature rising characteristic than samples 4 to 6. It is assumed that, in each of samples 7 to 18, the number of contact points between particles of the insulating powder 60 contributing to thermal conduction within the first region 46 was reduced, and thus, the rapid temperature rising characteristic was further improved.

[0068] In each of samples 13 to 18, the cumulative value of frequencies of particle diameters not smaller than 80 µm of the insulating powder 60 within the first region 46 was 0.1 to 5.0%. On the other hand, in each of samples 7 to 12, frequencies of particle diameters not smaller than 80 µm of the insulating powder 60 within the first region 46 were 0%. It has been found that samples 13 to 18 have more excellent rapid temperature rising characteristic than samples 7 to 12. It is assumed that, in each of samples 13 to 18, the number of contact points between particles of the insulating powder 60 contributing to thermal conduction within the first region 46 was further reduced, and thus, the rapid temperature rising characteristic was further improved.

[0069] Samples 6 to 18 are equal in terms of the median diameter of the particle diameters of the insulating powder 60 within the first region 46, and thus, as frequencies of particle diameters not smaller than 60 µm and frequencies of particle diameters not smaller than 80 µm of the insulating powder 60 within the first region 46 become higher, also frequencies of particles having small particle diameters become higher. As frequencies of particles having small particle diameters become higher, the number of contact points between particles is increased, but the particles having small particle diameters fill gaps between particles having large particle diameters, and thus, the gaps in the insulating powder 60 can be prevented from hindering thermal conduction. Since the increase in the frequencies of the particles having small particle diameters can be prevented from causing reduction in the coefficient of thermal conductivity, the coefficient of thermal conductivity can be improved by increasing the frequencies of the particles having large particle diameters.

[0070] As described above, although the present invention has been described based on the embodiments, the present invention is not limited to the above-described embodiments at all. It can be easily understood that various modifications can be devised without departing from the gist of the present invention. For example, the numeric values (e.g., the particle size distributions of the insulating powder 60 (FIGS. 3 and 4)) mentioned in the embodiments are exemplary, and, as a matter of course, other numeric values may be used. The positions and the lengths of the first region 46, 74 and the second region 47, 75 are exemplary, and may be set as appropriate. In addition, the shape of the tube 40, 71 is not particularly limited as long as the shape is tubular, and a cross section thereof orthogonal to the axial line O may have a circular shape, an elliptic shape, a polygonal shape, or the like.

[0071] In the embodiments, the case where the material of the front coil 51, 81 contains W or Mo as a main component has been described, but the present invention is not necessarily limited thereto. As a matter of course, a metal such as Fe, Ni, or Co, or an alloy containing any of these elements as a main component may be used as the material of the front coil 51, 81.

[0072] In the embodiments, the case where the material of the rear coil 52, 82 is a FeCr alloy or a NiCr alloy has been described, but the present invention is not necessarily limited thereto. As a matter of course, for example, pure Ni or a Co alloy (so-called control coil) for preventing over temperature rise in the front coil 51, 81 by controlling power to be supplied to the front coil 51, 81 may be used as the rear coil 52, 82.

[0073] In the embodiments, the case where a helical coil is use as the rear coil 52, 82 has been described, but the present invention is not necessarily limited thereto. As a matter of course, a conductor may be linearly disposed as the rear coil 52, 82. Alternatively, as a matter of course, the front coil 51 may be directly joined to the center shaft 20 with the rear coil 52 being omitted.

[0074] In the first embodiment, the case has been described where the outer diameter of each of the small-diameter portion 42 and the large-diameter portion 44 is uniform over the entire length thereof in the direction of the axial line O, but the present invention is not necessarily limited thereto. As a matter of course, the outer circumferential surface of at least one of the small-diameter portion 42 and the large-diameter portion 44 may be tapered so as to have a pyramid shape. In this case, the transitional portion 43 interposed between the small-diameter portion 42 and the large-diameter portion 44 may be omitted. In a case where the transitional portion 43 is not omitted, the shape of the transitional portion 43 may have a linear or tapered shape. Regardless of the shapes of the outer circumferential surfaces of the small-diameter portion 42 and the large-diameter portion 44, the thermal conductivity can be adjusted by adjusting the particle size distributions of the insulating powder 60 in the small-diameter portion 42 and the large-diameter portion 44.

[0075] In the embodiments, the case where the thickness of the tube 40, 71 is set to be uniform over the entire length thereof in the axial line O has been described, but the present invention is not necessarily limited thereto. As a matter of course, the thickness on the front side of the tube 40, 71 may be different from the thickness on the rear side of the tube 40, 71, in consideration of the heat capacity or the like of the tube 40, 71.

[Description of Reference Numerals]

[0076] 

10, 70: glow plug

40, 71: tube

41, 72: front end portion

42: small-diameter portion

44: large-diameter portion

45, 73: maximally heat-generating portion

46, 74: first region

47, 75: second region

50: coil

51, 81: front coil

52, 82: rear coil

60: insulating powder

O: axial line

Claims

1. A glow plug (10, 70) comprising:

a tube (40, 71) extending in a direction of an axial line (O) and having a closed front end;

a front coil (51, 81) disposed in the tube (40, 71) and having a front end which is electrically connected to a front end portion (41, 72) of the tube (40, 71), the front coil (51, 81) generating heat by being electrically energized;

a rear coil (52, 82) disposed in the tube (40, 71) and disposed rearward of the front coil (51, 81) so as to be electrically connected to the front coil (51, 81); and

insulating powder (60) sealed in the tube (40, 71), characterized in that:
a relationship is established where a median diameter of particle diameters of the insulating powder (60) disposed around the front coil (51, 81) and disposed within a first region (46, 74) is larger than a median diameter of particle diameters of the insulating powder (60) disposed around the rear coil (52, 82) and disposed within a second region (47, 75), the first region (46, 74) being a part in the direction of the axial line (O) of the tube (40, 71) and including a maximally heat-generating portion (45, 73) at which a temperature of a surface of the tube (40, 71) reaches a maximum temperature, the second region (47, 75) being a part in the direction of the axial line (O) of the tube (40, 71),
wherein:
in a particle size distribution of the insulating powder (60) disposed within the first region (46, 74), a cumulative value of frequencies of particle diameters not smaller than 60 µm is not lower than 1.0%, and wherein:
in a particle size distribution of the insulating powder (60) disposed within the second region (47, 75), frequencies of particle diameters not smaller than 60 µm are 0%.

2. The glow plug (10, 70) according to claim 1, wherein,
in the particle size distribution of the insulating powder (60) disposed within the first region (46, 74), a cumulative value of frequencies of particle diameters not smaller than 80 µm is not lower than 0.1%.

3. The glow plug (10, 70) according to claim 2, wherein,
in the particle size distribution of the insulating powder (60) disposed within the second region (47, 75), frequencies of particle diameters not smaller than 80 µm are 0%.

Ansprüche

1. Glühkerze (10, 70), umfassend:

ein Rohr (40, 71), das sich in eine Richtung von einer axialen Linie (O) erstreckt und ein geschlossenes vorderes Ende aufweist;

eine vordere Wendel (51, 81), die in dem Rohr (40, 71) angeordnet ist und ein vorderes Ende beinhaltet, das elektrisch mit einem vorderen Ende (41, 72) des Rohrs (40, 71) verbunden ist, wobei die vordere Wendel (51, 81) Wärme erzeugt, indem eine elektrische Spannung angelegt wird;

eine hintere Wendel (52, 82), die in dem Rohr (40, 71) angeordnet ist und hinter der vorderen Wendel (51, 81) angeordnet ist, um elektrisch mit der vorderen Wendel (51, 81) verbunden zu sein; und

isolierendes Pulver (60), das in dem Rohr (40, 71) abgedichtet ist, dadurch gekennzeichnet, dass:
eine Beziehung hergestellt wird, wo ein Mediandurchmesser der Teilchendurchmesser des isolierenden Pulvers (60), das um die vordere Wendel (51, 81) angeordnet ist und innerhalb eines ersten Bereichs (46, 74) angeordnet ist, größer ist als ein Mediandurchmesser der Teilchendurchmesser des isolierenden Pulvers (60), das um die hintere Wendel (52, 82) angebracht ist und innerhalb eines zweiten Bereichs (47, 75) angebracht ist, wobei der erste Bereich (46, 74) ein Teil in der Richtung der axialen Linie (O) des Rohrs (40, 71) ist und einen maximal wärmeerzeugenden Abschnitt (45, 73) umfasst, bei dem eine Temperatur einer Oberfläche des Rohrs (40, 71) eine Maximaltemperatur erreicht, wobei der zweite Bereich (47, 75) ein Teil in die Richtung der axialen Linie (O) des Rohrs (40, 71) ist,
wobei:
in einer Teilchengrößenverteilung des isolierenden Pulvers (60), das innerhalb des ersten Bereichs (46, 74) angebracht ist, ein kumulativer Wert von Frequenzen von Partikeldurchmessern, die nicht kleiner als 60 µm sind, nicht geringer als 1,0 % ist, und wobei:
in einer Teilchengrößenverteilung des isolierenden Pulvers (60), das innerhalb des zweiten Bereichs (47, 75) angebracht ist, Frequenzen von Partikeldurchmessern, die nicht kleiner als 60 µm sind, 0% betragen.

2. Glühkerze (10, 70) nach Anspruch 1, wobei
in der Teilchengrößenverteilung des isolierenden Pulvers (60), das innerhalb des ersten Bereichs (46, 74) angebracht ist, ein kumulativer Wert von Frequenzen von Partikeldurchmessern, die nicht kleiner als 80 µm sind, nicht geringer als 0,1% ist.

3. Glühkerze (10, 70) nach Anspruch 2, wobei
in der Teilchengrößenverteilung des isolierenden Pulvers (60), das innerhalb des zweiten Bereichs (47, 75) angebracht ist, Frequenzen von Partikeldurchmessern, die nicht kleiner als 80 µm sind, 0% betragen.

Revendications

1. Bougie de préchauffage (10, 70) comprenant :

un tube (40, 71) s'étendant dans la direction d'une ligne axiale (O) et ayant une extrémité avant fermée ;

une bobine avant (51, 81) disposée dans le tube (40, 71) et ayant une extrémité avant qui est connectée électriquement à une partie d'extrémité avant (41, 72) du tube (40, 71), la bobine avant (51, 81) générant de la chaleur en étant alimenté électriquement ;

une bobine arrière (52, 82) disposée dans le tube (40, 71) et disposée vers l'arrière de la bobine avant (51, 81) de manière à être connectée électriquement à la bobine avant (51, 81); et

une poudre isolante (60) scellée dans le tube (40, 71), caractérisée en ce que :
une relation est établie lorsqu'un diamètre médian des diamètres de particules de la poudre isolante (60) disposée autour de la bobine avant (51, 81) et disposée dans une première région (46, 74) est plus grand qu'un diamètre médian des diamètres de particules de la poudre isolante (60) disposée autour de la bobine arrière (52, 82) et disposée dans une deuxième région (47, 75), la première région (46, 74) faisant partie de la direction de la ligne axiale (O) du tube (40, 71) et incluant une partie génératrice de chaleur maximale (45, 73) à laquelle la température d'une surface du tube (40, 71) atteint une température maximale, la deuxième région (47, 75) faisant partie en direction de la ligne axiale (O) du tube (40, 71),
dans laquelle :
dans une distribution granulométrique de la poudre isolante (60) disposée dans la première région (46, 74), une valeur cumulée de fréquences de diamètres de particules non inférieurs à 60 µm n'est pas inférieure à 1,0%, et dans laquelle :
dans une distribution granulométrique de la poudre isolante (60) disposée dans la deuxième région (47, 75), les fréquences de diamètres de particules non inférieurs à 60 µm sont de 0%.

2. Bougie de préchauffage (10, 70) selon la revendication 1, dans laquelle,
dans la distribution granulométrique de la poudre isolante (60) disposée dans la première région (46, 74), une valeur cumulée de fréquences de diamètres de particules non inférieurs à 80 µm n'est pas inférieure à 0,1%.

3. Bougie de préchauffage (10, 70) selon la revendication 2, dans laquelle,
dans la distribution granulométrique de la poudre isolante (60) disposée dans la deuxième région (47, 75), les fréquences de diamètres de particules non inférieurs à 80 µm sont de 0%.

Drawing