Spark plug

Abstract

To provide a spark plug capable of securing voltage performance resistance of an insulator and improving thermal shock resistance.SOLUTION: A spark plug comprises: a cylinder-like insulation body that provides a shaft hole extended to a rear end side from a tip end side along a shaft line; a center electrode that is arranged into the shaft hole; a main metal tool that is arranged to an outer periphery of an insulation body; and a ground electrode of which one end part of itself is connected to the main metal tool and the other end part is opposite to the center electrode. The insulation body is projected from a tip end of the main metal tool by 1 mm or more. A thickness A of a part where a thickness of a diameter direction in the tip end of the insulation body becomes the maximum and a thickness B of a part where the thickness becomes the minimum satisfy 0.05≤(A-B) / B≤0.40, and the part where the thickness becomes the maximum is positioned within the range where the ground electrode is projected to a shaft line direction.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present invention relates to a spark plug. [Background technology]

[0002] In a spark plug having an insulator between a center electrode and a ground electrode, the technology disclosed in Patent Document 1 focuses mainly on the shape and dimensions of the center electrode in order to improve the voltage resistance performance of the insulator. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent No. 4879291 Summary of the Invention [Problem to be solved by the invention]

[0004] Spark plugs are required to have high voltage resistance as well as improved thermal shock resistance of the insulator.

[0005] The present invention has been made in response to this demand, and has an object to provide a spark plug which can ensure the voltage resistance performance of the insulator and improve the thermal shock resistance. [Means for solving the problem]

[0006] To achieve this object, the spark plug of the present invention comprises a cylindrical insulator having an axial hole extending from the front end to the rear end along the axis, a center electrode disposed in the axial hole, a metal shell disposed on the outer periphery of the insulator, and a ground electrode having one end connected to the metal shell and the other end facing the center electrode, the insulator protruding from the front end of the metal shell by 1 mm or more. The thickness A of the portion where the radial thickness is maximum at the front end of the insulator and the thickness B of the portion where the radial thickness is minimum satisfy 0.05≦(AB) / B≦0.40, and the portion where the thickness is maximum is located within a range obtained by projecting the ground electrode in the axial direction. Effect of the Invention

[0007] According to the present invention, the maximum radial thickness A and minimum radial thickness B at the tip of the insulator protruding 1 mm or more from the tip of the metal shell satisfy 0.05≦(AB) / A≦0.40. Since it is possible to prevent an extremely thin portion from being present at the tip of the insulator, the voltage resistance performance of the insulator can be ensured. The thickest portion of the insulator is disadvantageous in terms of thermal shock resistance compared to the thinner portions, but since the ground electrode is located close to the thickest portion and the cooling rate can be reduced, the thermal shock resistance can be improved. [Brief description of the drawings]

[0008] [Figure 1] 1 is a half-sectional view of a spark plug according to an embodiment; [Diagram 2] FIG. 2 is a partial cross-sectional view of a spark plug. [Diagram 3] FIG. 11 is a partial cross-sectional view of a spark plug according to a modified example. [Figure 4] 4(a) is a front view of the spark plug as viewed from the tip side, and (b) is a cross-sectional view of the spark plug taken along line IVb-IVb in FIG. [Diagram 5] FIG. 4 is a cross-sectional view of the spark plug taken along line IVb-IVb in FIG. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0009] Hereinafter, preferred embodiments of the present invention will be described with reference to the accompanying drawings. Fig. 1 is a half-sectional view of a spark plug 10 according to one embodiment, taken along an axis X. In Fig. 1, the lower side of the paper refers to the leading end side of the spark plug 10, and the upper side of the paper refers to the trailing end side of the spark plug 10 (the same applies to Figs. 2 and 3). As shown in Fig. 1, the spark plug 10 includes an insulator 11, a center electrode 16, a metal shell 18, and a ground electrode 28. The axis X is a straight line passing through a geometric center calculated by a known means when the outer shape of the cross section of the insulator 11 is made into a planar figure, and is a straight line passing through a plurality of geometric centers set for each of the cross sections.

[0010] The insulator 11 is a member made of ceramic such as alumina, which has excellent insulating properties and mechanical properties at high temperatures. The insulator 11 is provided with an axial hole 12 having a circular cross section. The axial hole 12 extends along the axis X from the front end to the rear end of the insulator 11. The insulator 11 has a cylindrical front end portion 13, a body portion 14 adjacent to the rear end of the front end portion 13 and thicker than the front end portion 13, and a protruding portion 15 protruding from the body portion 14 in a direction intersecting the axis X.

[0011] A cylindrical center electrode 16 is disposed in the axial hole 12 from the tip 13 to the body 14 of the insulator 11. The center electrode 16 has a core material with excellent thermal conductivity embedded in a base material. The base material is made of, for example, an alloy mainly made of Ni or Ni. The core material is made of, for example, an alloy mainly made of copper or copper. The core material can be omitted. The center electrode 16 has a tip made of a different material from the base material, for example a tip containing a precious metal such as Pt or Ir, disposed at the tip of the base material. The tip can be omitted.

[0012] The center electrode 16 is electrically connected to a terminal fitting 17 through a resistor 17a and conductive seals 17b and 17c in the axial hole 12 of the insulator 11. The terminal fitting 17 is a rod-shaped member to which a high-voltage cable or an ignition coil (neither shown) is connected. The material of the terminal fitting 17 is a conductive metal (such as low carbon steel).

[0013] The resistor 17a reduces radio noise when a spark occurs. The resistor 17a is formed of a composition containing, for example, glass particles as a main component, ceramic particles other than glass, and a conductive material. The conductive seal 17b fills the gap between the center electrode 16 and the resistor 17a, and the conductive seal 17c fills the gap between the resistor 17a and the terminal metal fitting 17. The conductive seals 17b and 17c are formed of a composition containing, for example, glass particles and metal particles.

[0014] The metal shell 18 is a substantially cylindrical member disposed on the outer periphery of the insulator 11. The material of the metal shell 18 is a metal having electrical conductivity (e.g., low carbon steel, etc.). The metal shell 18 has a body portion 19, a flange portion 20, a curved portion 21, a tool engagement portion 22, and a crimping portion 23 connected in this order from the front end side to the rear end side.

[0015] The body 19 has a shelf 24 on its inner periphery and a male thread 25 on its outer periphery. The shelf 24 is disposed on the tip side of the body 14 of the insulator 11 via a packing 26. The packing 26 is an annular plate material. The material of the packing 26 is a metal such as iron or steel that is softer than the metal material constituting the metal shell 18. The spark plug 10 is attached to a plug hole of an engine (not shown) by the male thread 25. The male thread 25 is generally standardized in JIS B8031:2006, for example, M12S, pitch 1.25 mm.

[0016] The outer diameter of the flange 20 adjacent to the rear end of the body 19 is larger than the outer diameter of the male thread 25. When the male thread 25 is tightened onto the threads of the plug hole, an axial force is generated in the male thread 25 by the flange 20. In this embodiment, the seating surface of the flange 20 is a surface perpendicular to the axis X, but depending on the shape of the plug hole, the seating surface may be a conical surface whose diameter decreases toward the tip side (so-called tapered seat type).

[0017] The curved portion 21 connects the flange portion 20 and the tool engagement portion 22. The curved portion 21 generates a force in a direction that moves the flange portion 20 and the tool engagement portion 22 away from each other in the axial direction due to the elastic force of bending deformation. The tool engagement portion 22 is a portion that is engaged with a tool such as a wrench when the male thread 25 is screwed into the thread of the plug hole. The crimped portion 23 is an annular portion that is bent radially inward. The crimped portion 23 is located on the rear end side of the protruding portion 15 of the insulator 11. A seal portion 27 filled with powder such as talc is provided between the protruding portion 15 of the insulator 11 and the crimped portion 23, around the entire circumference of the insulator 11.

[0018] The ground electrode 28 is a metal (e.g., nickel-based alloy) member connected to the body 19 of the metallic shell 18. The ground electrode 28 is a rod-shaped member and includes one end 29 connected to the body 19 and the other end 30 facing the center electrode 16. In this embodiment, the one end 29 of the ground electrode 28 is disposed substantially parallel to the axis X and curved toward the axis X from the one end 29 to the other end 30. A spark gap 31 (see FIG. 2) is provided between the other end 30 of the ground electrode 28 and the center electrode 16. The ground electrode 28 can have a tip disposed at the other end 30 made of a material different from the base material, for example, a tip containing a precious metal.

[0019] The spark plug 10 is manufactured, for example, by the following method. First, the center electrode 16 is inserted into the axial hole 12 from the rear end of the insulator 11, and is positioned so that the front end of the center electrode 16 appears outside the insulator 11. Next, the conductive seal 17b, resistor 17a, conductive seal 17c, and terminal fitting 17 are arranged in this order from the rear end of the insulator 11 into the axial hole 12, and the terminal fitting 17 and the center electrode 16 are electrically connected. Next, after a packing 26 is arranged on the shelf portion 24 of the metallic shell 18, the insulator 11 is inserted into the metallic shell 18, and the packing 26 is sandwiched between the body portion 14 of the insulator 11 and the shelf portion 24 of the metallic shell 18.

[0020] Next, after providing a seal portion 27 between the insulator 11 and the metal shell 18, the crimped portion 23 and the curved portion 21 are formed. As a result, the portion of the metal shell 18 from the shelf portion 24 to the crimped portion 23 applies an axial compressive load to the portion of the insulator 11 from the body portion 14 to the overhanging portion 15 via the packing 26 and the seal portion 27. As a result, the insulator 11 is held by the metal shell 18. The ground electrode 28 is bent to obtain the spark plug 10.

[0021] Fig. 2 is a partial cross-sectional view including the axis X of the spark plug 10. Fig. 2 illustrates the vicinity of the center electrode 16 protruding from the tip portion 13 of the insulator 11. The other end portion 30 of the ground electrode 28 faces the tip portion 32 of the center electrode 16, and a spark gap 31 is provided between the other end portion 30 of the ground electrode 28 and the tip portion 32 of the center electrode 16.

[0022] The tip 32 of the center electrode 16 is located further forward than the tip surface 33 of the tip portion 13 of the insulator 11. The tip surface 33 is an annular flat surface. The tip surface 33 is connected to a side surface 35 of the tip portion 13 via a rounded surface 34. The rounded surface 34 is a spherically curved surface, and the side surface 35 is a cylindrically curved surface with a constant outer diameter. The rounded surface 34 is a rounded portion between the tip surface 33 and the side surface 35 of the tip portion 13. The distance between the boundary 37 between the rounded surface 34 and the side surface 35 of the tip portion 13 and the axial hole 12 is referred to as the radial thickness T at the tip of the insulator 11.

[0023] Fig. 3 is a partial cross-sectional view including the axis X of a spark plug 10 in a modified example. The spark plug 10 shown in Fig. 2 has a side surface 35 of the tip portion 13 that is cylindrical in shape, whereas the modified example shown in Fig. 3 has a side surface 38 of the tip portion 13 that is conical in shape. In Fig. 3, the same reference numerals as in the spark plug 10 shown in Fig. 2 are used except for the side surface 38 of the tip portion 13, and the following description will be omitted.

[0024] Tip surface 33 of tip portion 13 is connected to side surface 38 via rounded surface 34. Side surface 38 is a conical surface whose outer diameter becomes smaller toward the tip. When side surface 38 of tip portion 13 is a conical surface, the distance between shaft hole 12 and circle 39 formed by an imaginary conical surface obtained by extending side surface 38 of tip portion 13 toward the tip side and an imaginary plane obtained by extending tip surface 33 of tip portion 13 radially outward intersect is called radial thickness T of insulator 11 at the tip.

[0025] 2 and 3, the distance D between the tip 36 of the body 19 of the metal shell 18 and the tip surface 33 of the tip portion 13 is 1 mm or more. That is, the insulator 11 protrudes from the tip 36 of the metal shell 18 by 1 mm or more.

[0026] Fig. 4(a) is a front view of the spark plug 10 as viewed from the tip side in the axial direction. Fig. 4(b) is a cross-sectional view of the spark plug 10 taken along line IVb-IVb in Fig. 2. Line IVb-IVb indicates a cut surface including the tip surface 33 of the insulator 11. For simplification, Figs. 4(a) and 4(b) omit the shelf portion 24, male thread 25, flange portion 20, and tool engagement portion 22 of the metal shell 18, which appear when the spark plug 10 is viewed from the tip side (the same applies to Fig. 5). The outline of the ground electrode 28 shown in Fig. 4(a) is shown by a two-dot chain line in Fig. 4(b).

[0027] 4(b), one end 29 of the ground electrode 28 has a rectangular cross section. The width L of the rectangular cross section of the ground electrode 28 is greater than its thickness S. The ground electrode 28 is disposed such that the width L of the one end 29 extends along the circumferential direction of the tip 36 of the body 19 (metal shell 18), and the thickness S of the one end 29 extends along the radial direction of the tip 36 of the body 19.

[0028] The radial thickness T of the insulator 11 (tip portion 13) at the tip is the length of a line segment cut by the axial hole 12 and the boundary 37 of a straight line 41 passing through the center 40 of the axial hole 12. When the shape of the side surface 38 of the tip portion 13 of the insulator 11 is a cone surface as shown in Fig. 3, the radial thickness T of the insulator 11 at the tip is the length of a line segment cut by the axial hole 12 and a circle 39 of the straight line 41 passing through the center 40 of the axial hole 12. In the spark plug 10, since the center 40 of the axial hole 12 of the insulator 11 is not on the axis X, a portion 42 where the thickness T is maximum and a portion 43 where the thickness T is minimum are present in the insulator 11.

[0029] When the thickness T of the portion 42 is the maximum thickness A and the thickness T of the portion 43 is the minimum thickness B, the spark plug 10 satisfies 0.05≦(AB) / B≦0.40. Preferably, it satisfies 0.10≦(AB) / B≦0.40, and more preferably, it satisfies 0.26≦(AB) / B≦0.40. Furthermore, the portion 42 where the thickness T is maximum is located within a range in which the ground electrode 28 is projected from the axial direction (a range surrounded by a two-dot chain line in FIG. 4(b)). It can also be said that the portion 42 where the thickness T is maximum is located inside a range (a portion of width L) in which one end 29 of the ground electrode 28 is projected toward the axis X.

[0030] The spark plug 10 is attached to a plug hole of an engine (not shown) by a male thread 25 of a metal shell 18. In a four-stroke engine, the temperature of the engine's combustion chamber rises during the expansion stroke and drops during the intake stroke. A portion of the insulator 11 that protrudes 1 mm or more from the tip 36 of the metal shell 18 appears in the combustion chamber, so a large temperature change occurs between the expansion stroke and the intake stroke. The insulator 11 is required to have thermal shock resistance so that it will not be destroyed by this temperature change.

[0031] The portion of the insulator 11 with a small thickness T is advantageous in terms of thermal shock resistance, but disadvantageous in terms of voltage resistance, compared to the portion of the insulator 11 with a large thickness T. In order to ensure the voltage resistance performance of the insulator 11, the radial thickness of the insulator 11 is set to satisfy 0.05≦(AB) / B≦0.40, and an extremely thin portion of the thickness T is not present at the tip of the insulator 11. Furthermore, since the electric field strength is generally high at the connection point between the one end 29 of the ground electrode 28 and the metal shell 18, the portion 42 of the insulator 11 with the maximum thickness T is placed close to the one end 29 of the ground electrode 28 to reduce the occurrence of dielectric breakdown. This ensures the voltage resistance performance of the insulator 11.

[0032] The portion 42 of the insulator 11 where the thickness T is maximum is disadvantageous in terms of thermal shock resistance compared to portions of the insulator 11 where the thickness T is smaller. Therefore, by bringing one end 29 of the ground electrode 28 close to the portion 42 of the insulator 11 where the thickness T is maximum, for example, the ground electrode 28 blocks the airflow in the combustion chamber or blocks the injected fuel, making the portion 42 less susceptible to the effects of cooling during the intake stroke. Compared to a case in which the portion 42 is located outside the range projected by the ground electrode 28, the cooling rate of the portion 42 during the intake stroke can be made smaller, thereby improving the thermal shock resistance of the insulator 11.

[0033] 5 is a cross-sectional view of the spark plug 10 taken along line IVb-IVb in FIG. 2, seen from the tip end side in the axial direction. The diameter of the center electrode 16 is slightly smaller than the diameter of the axial hole 12, so that a gap 44 is formed between the inner circumference of the insulator 11 defined by the axial hole 12 and the center electrode 16. In FIG. 5, a position 45 where the gap 44 between the inner circumference (axial hole 12) of the insulator 11 and the center electrode 16 is smallest is preferably set so that an angle θ between a line segment 46 connecting the position 45 to the center 40 of the axial hole 12 and a line segment 48 connecting the center 47 of the ground electrode 28 to the center 40 is 30° or less. The center 47 of the ground electrode 28 is the geometric center calculated by a known means when the cross section of the ground electrode 28 is a plane figure. θ includes 0°.

[0034] Since the thermal conductivity of the center electrode 16 is superior to that of the insulator 11, the position 45 where the gap 44 between the axial hole 12 of the insulator 11 and the center electrode 16 is smallest has a greater effect of cooling the insulator 11 by heat transfer from the center electrode 16 than the portion where the gap 44 is larger. This prevents the temperature of the insulator 11 near the position 45 from becoming excessively high. If the angle θ between the line segment 46 and the line segment 48 is 30° or less, the position 45 is located near the portion 42 of the insulator 11 where the thickness T is maximum, so that the portion 42 is cooled by the center electrode 16. Since the temperature of the portion 42 heated during the expansion stroke can be lowered, the temperature change of the portion 42 can be reduced. Since the thermal shock resistance of the portion 42 can be improved, the thermal shock resistance of the insulator 11 can be further improved. EXAMPLES

[0035] The present invention will be described in more detail with reference to examples, but the present invention is not limited to these examples.

[0036] (Test 1) The tester prepared components of the spark plug 10 in the embodiment, including an insulator 11 made of ceramic containing alumina as a main component, with a diameter of 3.85 mm (median) at the boundary 37 of the tip portion 13 and a diameter of 2.15 mm (median) at the axial hole 12 of the tip surface 33, a center electrode 16 with a diameter of 2.00 mm, a metal shell 18 with a nominal diameter of 12 mm for the male thread 25, and a ground electrode 28 with a rectangular cross-sectional width L of 2.7 mm and a thickness S of 1.3 mm. The insulators 11 prepared had different distances between the center of the axial hole 12 at the tip surface 33 and the axis X.

[0037] The tester prepared Sample No. 1-24, which had different distances D between the tip 36 of the metal shell 18 and the tip face 33 of the insulator 11, and conducted a test to examine the effect of the relationship between the maximum thickness A and the minimum thickness B of the insulator 11 on the voltage resistance and thermal shock resistance. The positional relationship between the axial hole 12 and the center electrode 16 of Sample No. 1-24 was always an angle θ>30°.

[0038] For the voltage resistance test, a sample was attached to an engine placed on a test bench, and the engine was operated for 10 minutes under the following conditions to check the condition of the sample. The engine was a turbocharged inline 4-cylinder gasoline direct injection engine with a displacement of 1600cc, with a rotation speed of 3000rpm, an indicated mean effective pressure (NMEP) of 230kPa, an ignition timing of 0 degrees before top dead center (TDC), and an air-fuel ratio of 10.5. For the voltage resistance, samples in which the insulator 11 broke down and the engine did not operate normally during the test were judged as B, and samples in which the engine operated normally were judged as A.

[0039] For the thermal shock resistance test, a sample was attached to an engine installed on a test bench, and the engine was operated under the following conditions to check the condition of the sample: The engine was a 2000cc inline 4-cylinder gasoline direct injection engine with a turbocharger and an air-fuel ratio of 10.5, and with the transmission set to second gear, the engine speed was increased from 2000 rpm to 6000 rpm in 10 seconds, and then decreased from 6000 rpm to 2000 rpm in 15 seconds, with this process being one cycle, and 20 cycles were repeated.

[0040] For thermal shock resistance, samples in which insulation 11 broke down and the engine stopped working properly during the test were rated D, samples in which the engine worked properly during the test but a crack occurred in insulator 11 with a length of 30% or less of the thickness of insulator 11 was rated C, samples in which the engine worked properly during the test but a crack occurred in insulator 11 with a length of 10% or less of the thickness of insulator 11 was rated B, and samples in which the engine worked properly during the test and no cracks occurred in insulator 11 were rated A.

[0041] [Table 1]

[0042] For sample No. 1-24, the distance D between the tip 36 of the metal shell 18 and the tip surface 33 of the insulator 11, the maximum thickness A, the minimum thickness B, (AB) / B, the positional relationship between the part 42 of the insulator 11 where the thickness T is maximum and the range of the ground electrode 28 projected from the axial direction, and the test results are shown in Table 1. The quotient for (AB) / B is rounded off to two decimal places. In the "electrode position" column of Table 1, if the part 42 of the insulator 11 where the thickness T is maximum is within the range of the ground electrode 28 projected from the axial direction, it is marked "inside", and if it is not within the range, it is marked "outside".

[0043] For No. 1-4, all (AB) / B=0, but the distance D was different. For No. 1, D=0.5mm and the thermal shock resistance was judged to be C, but for No. 2-4, the distance D was 1.0-3.5mm and the thermal shock resistance was judged to be D. According to No. 1-4, it was found that when (AB) / B=0, the boundary for distance D at which the thermal shock resistance rating changes from C to D is D=1.0mm.

[0044] For both Nos. 7 and 8, D = 1.0 mm and (AB) / B = 0.05, but for No. 7, there was no portion 42 within the range of the ground electrode 28 projected from the axial direction, while for No. 8, there was portion 42 within the range of the ground electrode 28 projected from the axial direction. The thermal shock resistance was judged to be C for No. 8, but D for No. 7. When (AB) / B<0.05 as in Nos. 5 and 6, the thermal shock resistance was judged to be D even if there was portion 42 within the range of the ground electrode 28 projected from the axial direction. According to Nos. 5-8, it was found that when there is portion 42 within the range of the ground electrode 28 projected from the axial direction, the (AB) / B boundary at which the thermal shock resistance rating changes from D to C is 0.05.

[0045] On the other hand, the voltage withstand rating of No. 1-22 was A, and No. 23 and No. 24 were B. According to No. 1-24, when the portion 42 is present within the range of the ground electrode 28 projected from the axial direction, if 0.05≦(AB) / B≦0.40, the voltage withstand rating can be A, and the thermal shock resistance can be A, B, or C.

[0046] For both No. 11 and No. 12, D = 1.0 mm and (AB) / B = 0.10, but for No. 11, there was no portion 42 within the range of the ground electrode 28 projected from the axial direction, and for No. 12, there was a portion 42 within the range of the ground electrode 28 projected from the axial direction. The thermal shock resistance was judged as D for No. 11, but B for No. 7. When (AB) / B<0.10 as in No. 8-10, even if there was a portion 42 within the range of the ground electrode 28 projected from the axial direction, the thermal shock resistance was judged as C. According to No. 8-24, it was found that when there is a portion 42 within the range of the ground electrode 28 projected from the axial direction, the thermal shock resistance can be judged as A or B if 0.10≦(AB) / B≦0.40.

[0047] For both No. 17 and No. 18, D≧1.0 mm and (AB) / B=0.26, but for No. 17, there was no portion 42 within the range of the ground electrode 28 projected from the axial direction, while for No. 18, there was a portion 42 within the range of the ground electrode 28 projected from the axial direction. The thermal shock resistance was judged as D for No. 17, but A for No. 18. When (AB) / B<0.26 as in No. 12-16, even if there was a portion 42 within the range of the ground electrode 28 projected from the axial direction, the thermal shock resistance was judged as B. According to No. 12-24, when there was a portion 42 within the range of the ground electrode 28 projected from the axial direction, if 0.26≦(AB) / B≦0.40, the thermal shock resistance could be judged as A.

[0048] (Test 2) The tester prepared the same parts as the samples in Test 1, and produced Samples No. 25-36, which differ in the angle θ, distance D, and (AB) / B between the line segment 46 connecting the position 45 where the gap 44 is smallest and the center 40 of the axial hole 12, and the line segment 48 connecting the center 40 and the center 47 of the ground electrode 28. In all of Samples No. 25-36, the part 42 of the insulator 11 where the thickness T is maximum is within the range of the ground electrode 28 projected from the axial direction. Samples No. 25-36 were subjected to the same thermal shock resistance test as Test 1. The criteria for thermal shock resistance were the same as those in Test 1.

[0049] [Table 2]

[0050] For sample No. 25-36, the distance D, maximum thickness A, minimum thickness B, (AB) / B, the positional relationship between the part 42 of the insulator 11 where the thickness T is maximum and the area where the ground electrode 28 is projected from the axial direction, the angle θ, and the test results are shown in Table 2. (AB) / B is rounded off to two decimal places.

[0051] In No. 25-27 with (AB) / B=0.05, No. 27 with angle θ=30° was judged as B for thermal shock resistance, but No. 25 and 26 with angle θ>30° were judged as C for thermal shock resistance. In No. 28-30 with (AB) / B=0.10, No. 30 with angle θ=30° was judged as A for thermal shock resistance, but No. 28 and 29 with angle θ>30° were judged as B for thermal shock resistance. In No. 31-36 with (AB) / B=0.23, No. 33 and 36 with angle θ=30° were judged as A for thermal shock resistance, but No. 31, 32, 34 and 35 with angle θ>30° were judged as B for thermal shock resistance. Table 2 shows that thermal shock resistance can be improved when angle θ≦30°.

[0052] The present invention has been described above based on the embodiments, but the present invention is not limited to the above embodiments, and it can be easily inferred that various improvements and modifications are possible within the scope of the present invention without departing from the spirit of the present invention.

[0053] In the embodiment, a case has been described in which the corners where the tip surface 33 of the insulator 11 connects to the side surfaces 35, 38 are rounded and the tip surface 33 and the side surfaces 35, 38 are connected by a spherical round surface 34, but this is not necessarily limited to this. It is of course possible to chamfer the corners where the tip surface 33 of the insulator 11 connects to the side surfaces 35, 38 and connect the tip surface 33 and the side surfaces 35, 38 by a cone-shaped corner surface. The thickness T of the insulator 11 when the tip surface 33 and the side surfaces 35, 38 are connected by a corner surface is specified in the same way as the thickness T of the insulator 11 in the embodiment.

[0054] It is also possible to omit rounding or chamfering of the corners where the tip surface 33 of the insulator 11 is connected to the side surfaces 35, 38. When the tip surface 33 is connected to the side surfaces 35, 38 without any rounded or angular surfaces, the thickness T of the insulator 11 is the distance between the corners connecting the tip surface 33 to the side surfaces 35, 38 and the shaft hole 12.

[0055] In the embodiment, the angle θ between the line segment 46 and the line segment 48 is specified in the cross-sectional view of the spark plug 10 viewed from the tip side in the axial direction using a cut surface (line IVb-IVb) including the tip surface 33 of the insulator 11, but this is not necessarily limited to this. It is of course possible to specify the angle θ between the line segment 46 and the line segment 48 in a surface (cross section) that is parallel to the tip surface 33 of the insulator 11 and in which the ground electrode 28 and the gap 44 appear. In other words, the surface that includes the tip surface 33 of the insulator 11 is included in the plane parallel to the tip surface 33 of the insulator 11.

[0056] In the embodiment, a case has been described in which line segment 48 exists in a positive position clockwise with respect to line segment 46 (above line segment 46 in FIG. 5), but this is not necessarily limited to this. Line segment 48 may also exist in a negative position clockwise with respect to line segment 46 (below line segment 46 in FIG. 5). Even when line segment 48 exists in a negative position clockwise with respect to line segment 46, the same effects as those of the embodiment can be achieved as long as the angle θ between line segment 46 and line segment 48 is 30° or less (including 0°). [Explanation of symbols]

[0057] 10 Spark plug 11 Insulators 12 Shaft hole (inner circumference of insulator) 16 Center electrode 18 Metal fitting 28 Ground electrode 29 One end 30 Other end 33 Tip surface of insulator 36 Tip of main body 40 Center of shaft hole 42 Maximum thickness 43 Minimum thickness 44 Gap 45 Position where the gap is smallest 46 The line segment connecting the position where the gap is smallest and the center of the shaft hole 47 Center of gravity of ground electrode 48 Line segment connecting the center of gravity of the ground electrode and the center of the shaft hole X axis θ Angle

Claims

1. a cylindrical insulator having an axial hole extending from a front end side to a rear end side along an axis; a center electrode disposed within the axial hole; A metal shell disposed on an outer periphery of the insulator; a ground electrode having one end connected to the metallic shell and the other end facing a tip of the center electrode, The insulator protrudes from the tip of the metallic shell by 1 mm or more, the maximum thickness of the radial thickness at the tip of the insulator is A and the minimum thickness of the radial thickness at the tip of the insulator is B, the relationship 0.05≦(A−B) / B≦0.40 is satisfied; The maximum portion is located within a range obtained by projecting the ground electrode in an axial direction of the spark plug.

2. 2. The spark plug according to claim 1, wherein 0.10≦(A−B) / B≦0.

40.

3. 2. The spark plug according to claim 1, wherein 0.26≦(A−B) / B≦0.

40.

4. In a plane parallel to the tip surface of the insulator, 4. The spark plug according to claim 1, wherein an angle formed by a line segment connecting a position where a gap between an inner periphery of the insulator and the center electrode is smallest and a center of the axial hole and a line segment connecting the center of gravity of the ground electrode and the center is equal to or smaller than 30°.

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