Spark plug for internal combustion engine

Abstract

 A spark plug for an internal combustion engine is comprised of a center electrode (3), a cylindrical insulator (2) holding the center electrode in a center hole thereof, a cylindrical metal housing (1) holding the insulator therein, a first earth electrode (4) having a free end disposed opposite the center electrode's edge to form a first discharge gap A, a second earth electrode (5, 6) having a free end disposed opposite the center electrode's side-surface to form a second discharge gap. The free end of the second earth electrode has a cross-sectional area S1 at a portion normally opposite to the insulator's side surface. The insulator's side surface has a projected area S2 to which the free end of the second earth electrode is normally projected. The free end of the second earth electrode is disposed at a portion outside the outside diameter B of the insulator's edge. There is the following relationship between the first discharge gap A and the outside diameter B: B . 5A - 2.5 (mm).

Description

[0001] The present invention relates to a spark plug for an internal combustion engine, particularly, to an arrangement for preventing fuel-bridges from forming across a discharge gap of a spark plug.

[0002] An ordinary spark plug for an internal combustion engine is comprised of a center electrode, an insulator that covers and holds the circumference of the center electrode, and an earth electrode having one end fixed to a metal housing and the free end that forms a discharge gap between the same and the edge of the center electrode. A spark is generated across the discharge gap to ignite air-fuel mixture in an engine combustion chamber. Recently, environmental concern has been growing, and stratified fuel combustion system has been adopted to provide an environmentally suitable internal combustion engine of low fuel consumption.

[0003] However, if such stratified fuel combustion is intended in a combustion chamber of an engine, high ratio of fuel-air mixture has to be gathered around the spark plug. This may increase carbon deposits. The carbon deposits may cause deterioration of the insulation performance of the surface of the insulator. Accordingly, a spark from the center electrode through the surface of the insulator to the inside of the metal housing is generated instead of the normal spark between the center electrode and the earth electrode, causing ignition problems of the engine.

[0004] In order to improve the above problems, JP-Y2-53-41629 and JP-A-47-19236 disclose self-cleaning spark plugs. Each of the disclosed spark plugs (hereinafter referred to as the double-earth-electrode type spark plug) is comprised of a first earth electrode (main earth electrode), a first discharge gap formed by the first earth electrode, and a second earth electrode (auxiliary earth electrode) having a fixed end fixed to the metal housing and a free end forming a second discharge gap between the same and the side surface of the center electrode.

[0005] In the double-earth-electrode type spark plug, the second of the second earth electrode opposite the side surface of the center electrode is disposed at the outside of the outside diameter of the insulator. In JP-Y2-53-41629, normal spark discharges are generated across the first discharge gap, while the spark discharge is generated across the second discharge gap to burn away carbon deposites covering the insulator, thereby preventing the spark from discharging into the inside of the insulator as well as preventing the ignition performance from decreasing.

[0006] However, in the above-described double-earth-electrode type self-cleaning spark plug, fuel such as gasoline remains in the first and second discharge gaps and forms a fuel-bridge. This causes engine misfires or ignition failure. Formation of the fuel-bridge is illustrated in Figs.10A - 10C. In the figures, J1 represents a center electrode, J2 is an insulator, J3 is a metal housing, J4 is a first earth electrode, J5 and J6 are, respectively, second earth electrodes. Fuel is illustrated by hatched portions.

[0007] As shown in Fig. 10A, fuel-bridge j7 is formed in a first discharge gap (main gap) between first earth electrode J4 and center electrode J1 and fuel-bridge J8, is formed in a second discharge gap (auxiliary gap) between second earth electrodes J5, J6 and the center electrode. If both the gaps are sufficiently wide, the bridges J7, J8 are easily broken due to vehicle vibration or vibration caused by pressure change of combustion chambers. However, if the first discharge gap is excessively wide, discharge voltage has to be increased. If the second discharge gaps become excessively wider, sparks are not generated across the gap but generated along the surface of the insulator to reach the inside of the metal housing.

[0008] As shown in Figs. 10B and 10C, even if the fuel-bridge crossing the second discharge gap is broken due to the vibration or the like, the fuel moves to the first discharge gap to form another fuel-bridge J7. Thus, in the double-earth-electrode type spark plug, it is more difficult to prevent the fuel-bridge or the misfires than to prevent the fuel-bridge in a normal type spark plug that has no such second electrode.

[0009] This kind of the fuel-bridge is often formed if fuel mixture becomes rich in case an internal combustion engine is started under a cold temperature. The fuel-bridge is also formed in the stratified combustion engine that burns excessively rich fuel-air mixture.

[0010] In view of the above problem, an object of the invention is to prevent the fuel-bridges from forming at the first discharge gap and the second discharge gap of a double-earth-electrode type spark plug for an internal combustion engine, while maintaining sufficient ignition performance.

[0011] As described above fuel is apt to remain at the gap between the second earth electrodes J5, J6 and a side surface of the insulator J2 right opposite those electrodes. Therefore, the invention has been made by discovery of the relationship between the second earth electrodes and the surface of the insulator opposite thereto and the relationship between the second earth electrodes and a side surface of the center electrode.

[0012] According to a feature of the invention, there is the following relationship between the first discharge gap A and the outside diameter B of the insulator's edge: B . 5A - 2.5 (mm). In addition, distance C (mm) between the free end of the second earth electrode and the insulator's side surface and a ratio of a projected area S2 of the insulator's side surface to which a portion of the cross-sectional area S1 of the second earth electrode are set within a range defined by lines connecting the following points when the distance C is plotted on the x-axis and the ratio is plotted on the y-axis:

(C, S2/S1) = (0.3, 0)

(C, S2/S1) = (1.2, 0)

(C, S2/S1) = (1.2, 1.0)

(C, S2/S1) = (0.8, 1.0)

(C, S2/S1) = (0.3, 0.5)

[0013] The above range is shown in Figs. 7 and 8. According to the invention, the fuel-bridges are prevented from forming at the first discharge gap and second discharge gap while normal ignition performance is maintained.

[0014] In the portion where the free end of the second earth electrode and the side surface of the insulator's edge face each other (the overlapping portions in the axial direction of the spark plug), the surfaces facing each other are not flat but slightly curved. Therefore, the areas S1, S2 of flat surfaces approximately represent the facing surface areas.

[0015] According to another feature of the invention, the distance C and the ratio S2/S1 are set within a range defined by lines connecting the following points:

(C, S2/S1) = (0.3, 0)

(C, S2/S1) = (1.2, 0)

(C, S2/S1) = (1.2, 1.0)

(C, S2/S1) = (1.0, 1.0)

(C, S2/S1) = (0.3, 0.3)

[0016] This effectively prevents the fuel-bridges from forming at the first and second discharge gaps.

[0017] According to a further feature of the invention, the insulator's side surface has a projected area S2 to which a portion of the free end of the second earth electrode is normally projected, and the first discharge gap A, the outside diameter B, the cross-sectional area S1, the projected area S2, and the distance C are respectively determined so that a fuel-bridge is not formed at the first and second discharge gaps when the following test is conducted:

placing the spark plug under temperature of about -25°;,

dipping the spark plug in fuel of an internal combustion engine;

subsequently naturally dropping the spark plug by a prescribed distance with the side of the center edge being down;

holding the spark plug in the air.

[0018] Therefore, the performance of a spark plug is confirmed by a method for providing practically the same outside stress as the vibration applied to the spark plug.

[0019] In view of the practical level of the vibration, it is preferable that the drop distance is equal to or less than 4 cm.

[0020] It is also preferable that the distance C is equal to or longer than 0.3 mm or equal to or shorter than 1.2 mm.

[0021] It is also preferable that the first discharge gap A is equal to or longer than 0.7 mm or equal to or shorter than 1.3 mm.

[0022] If the distance C and the gap A are shorter than the shortest limits, the engine idling operation may become rough. On the other hand, if longer than the longest limits, the spark discharge cannot be generated, resulting in misfires.

[0023] In addition, it is preferable that the outside diameter B of the center electrode's edge is equal to or larger than 0.3 mm or equal to or less than 2.8 mm.

[0024] Other objects, features and characteristics of the present invention as well as the functions of related parts of the present invention will become clear from a study of the following detailed description, the appended claims and the drawings. In the drawings:

Fig. 1 is a half-cross-sectional view of a spark plug according to a preferred embodiment of the invention;

Fig. 2 is an enlarged fragmentary view of a portion indicated by arrow X in Fig. 1;

Fig. 3 is a portion indicated by arrow Y in Fig. 1;

Figs. 4A and 4B are diagram showing various dimensions of the spark plug shown in Fig. 1;

Figs. 5A and 5B are schematic diagrams illustrating a test of fuel holding force applied to the above embodiment;

Figs. 6A, 6B and 6C are diagrams illustrating variations of projected surface area S2;

Fig. 7 is a graph showing a test result of the fuel holding force at a second discharge gap;

Fig. 8 is a graph showing a test result of the fuel holding force at a first discharge gap;

Fig. 9 is a diagram illustrating a variation of the above embodiment; and

Figs. 10A, 10B and 10C are diagrams illustrating a process of forming a fuel-bridge.

[0025] The embodiments according to the invention will be described with reference to the appended drawings.

[0026] As shown in Fig. 1, spark plug 100 has cylindrical metal housing 1, which is provided with fastening bolt 1a at the outer periphery to be fastened to an engine block. Insulating member 2, which is made of such as aluminum ceramics (A1₂O₃), is fixed to the inside of metal housing 1. Insulator 2 has end portion 2a extending outward from metal housing 1. As shown in Fig. 2, end portion 2a has cylindrical edge portion 2b that has a smaller outside diameter than the other portion thereof.

[0027] Center electrode 3 is fixed to center hole 2c of insulator 2 and is held by metal housing 1 via insulator 2. Center electrode 3 is a double-layered columnar member that is comprised of an internal member made of copper (Cu) or another heat conductive material and an external member made of nickel (Ni) alloy or another heat and corrosion resistant material. The edge of center electrode 3 extends outward from end portion 2a of insulator 2. As shown in Fig. 2, base portion 3a of center electrode 3 has cylindrical edge portion 3b that is smaller in outside diameter than the other portions thereof. The outer periphery of edge portion 3b is located inside of the inner periphery of edge portion 2b of insulator 2.

[0028] As shown in Figs. 2 and 3, first earth electrode 4 and second earth electrodes 5 and 6 are fixed, by such as a welder, to an end of metal housing 1. These first and second earth electrodes 4-6 are made of a nickel alloy. The end (free end) of first earth electrode opposite to the end (fixed end) fixed to metal housing 1 is disposed opposite the top of edge portion 3b of center electrode 3, forming first discharge gap A between the same and the edge portion 3b.

[0029] The free ends of second earth electrodes 5 and 6 opposite to the fixed ends fixed to metal housing 1 are disposed opposite side surfaces of edge portion 3b of center electrode 3, forming second discharge gap. The free ends of second earth electrodes 5, 6 are disposed outside the diameter of the outer periphery of insulator's edge portion 2a by a distance C (shown in Fig. 4).

[0030] Dimensions A, B, C, S1 and S2, shown in Fig. 4, are specifically defined. Dimension A is the width of first discharge gap, dimension B is the outside diameter of edge portion 3b of center electrode 3 (hereinafter referred to as the center electrode diameter). Dimension C is a distance between second electrodes 5, 6 and the side surfaces of edge portion 2b of insulator 2 in the width direction (horizontal direction in Fig. 4) of second discharge gap.

[0031] The sectional area of the free end of second earth electrodes 5 and 6 in the direction perpendicular to the width direction of the discharge gap is assumed to be S1. Sectional area S1 is the sectional area of the second earth electrodes 5 and 6 disposed at imaginary surface 900 (the surface perpendicular to the width direction of the second discharge gap) indicated by a one-dot chain line in Fig. 4A. Portions of the free end surfaces of second earth electrodes 5, 6 and the side surfaces of edge portion 2b of center electrode 2 overlap each other in the axial direction to form confronting domain 901. In other words, portions of the free end's surfaces are projected on the side surface of edge portion 2b to form a projected surface (901).

[0032] In confronting domain 901, the area of the free end's surfaces of second earth electrodes 5 and 6 normally projected to the side surfaces of edge portion 2b of insulator 2 is assumed to be S2. Therefore, sectional area S1 and projected surface area S2 are areas of rectangular surface. As shown in Fig. 4B, the surfaces of insulator 2 and second earth electrodes 5 and 6 in confronting domain 901 are not flat but are slightly curved. However, they are approximated to flat surface areas S1 and S2.

[0033] In spark plug 100, the following fuel holding capacity tests were conducted. As shown in Fig. 5A, fuel (high-octane gasoline) 910 of an internal combustion engine is put in a cold chamber (such as a beaker) of -25°C so that the temperature of fuel 910 becomes -25°C and the viscosity thereof increases to easily form fuel-bridges.

[0034] In this cold chamber, spark plug 100 is hung with a string 912 that is fixed to the end opposite earth electrodes 4-6. Then, earth electrodes 4-6 and the front end of metal housing 1 are dipped in fuel 910 and taken out from the same. The fuel-bridges are formed at first discharge gap A between first earth electrode 4 and center electrode 3 and second discharge gaps B between respective second earth electrodes 5, 6 and center electrode 3. These discharge gaps are short-circuited by fuel 910.

[0035] Subsequently, as illustrated in Fig. 5B, spark plug 100 is dropped by a predetermined distance H under natural conditions with the edge-side of center electrode 3 being down until it is stopped by string 912. Accordingly, the fuel-bridges at the discharge gaps can be broken due to a shock caused when spark plug is stopped. If a holding force of the fuel is large enough, the fuel-bridge remains at any of the discharge gaps. If the holding force is not large enough, the fuel-bridge cannot remain there. The dropping height H was changed from 0 cm to 1cm, 2 cm, 3cm, 4 cm, and 5 cm.

[0036] At first, the fuel holding forces of the second discharge gaps are tested while various dimensions including the cross-sectional area S1, projected surface area S2, and distance C are changed. In this test, the following three different cross-sectional areas S1 of the second earth electrodes are tested: 1.44 mm² (0.8 mm × 0.8 mm), 2.64 mm² (1.2 mm × 2.2 mm), 4.16 mm² (1.6 mm × 2.6 mm).

[0037] As shown in Figs. 6A, 6B, and 6C, the free end of second earth electrodes 5 and 6 and the side surfaces of edge portion 2b of insulator 2 are shifted in the axial direction of the spark plug so that the above projected surface area S2 can be changed. This changes the ratio S2/S1 between 0 and 1. For example, in Fig. 6A, ratio S2/S1 is 0. In Fig. 6B, ratio S2/S1 is larger than 0 and smaller than 1. In Fig. 6C ratio S2/S1 is 1.

[0038] The above distance C was changed to values between 0.3 mm and 1.2 mm in order to keep sufficient ignition performance of the second discharge gap. If distance C is equal to or less than 0.3mm, sparks are apt to be discharged across the second discharge gap B rather than the first discharge gap A. This may cause a rough idling operation. On the other hand, if the distance C is longer than 1.2mm, it is difficult to generate the spark across the second discharge gap. This hinders the surface discharge and causes engine stall when carbon deposits form on the surface of spark plug 100.

[0039] In the above fuel holding test, a test sample is determined to be "no good", if the fuel-bridge remains at the second gaps of the test sample after the dropping under natural condition. On the other hand, if the fuel-bridge does not remain at the second discharge gaps, the sample is determined to be " good". If the test sample is determined to be " good" where the dropping height H is equal to or less than 4 cm, it is practically considered that the fuel-bridge is not formed. This was confirmed after various spark plugs having the different ratio S2/S1 and the different distance C were tested.

[0040] The fuel holding force at second discharge gap of the spark plugs were tested under the various heightes H being less than 4 cm. The relationship between the height H and the ratio S2/S1 found in the above test is shown in Fig. 7. Fig. 7 is a graph of rectangular coordinates having x-axis indicating the above distance C and x-axis indicating the above ratio S2/S1. In Fig. 7, pentagonal area R with diagonal hatching, in other words, the area R (and lines) defined by straight lines connecting respective points P1 - P5 shows the area of the relationship between the distance C and areas S1 and S2 ("good" range of second discharge gap) where the above "good" result can be expected when the height H is equal to or less than 4 cm. The points P1 - P5 are located as follows:

(C, S2/S1) = (0.3, 0)

(C, S2/S1) = (1.2, 0)

(C, S2/S1) = (1.2, 1.0)

(C, S2/S1) = (0.8, 1.0)

(C, S2/S1) = (0.3, 0.5)

[0041] The above result was obtained after three time tests. Range R is the range of the second discharge gap at least one "good" result is obtained. Most preferable range of the second discharge gap is the pentagonal range that is defined by straight lines connecting each of the points P1 - P3, P'4, P'5.

[0042] The points P4' and P5' are located as follows:

(C, S2/S1) = (1.0, 1.0)

(C, S2/S1) = (0.3, 0.3)

[0043] Therefore, if the above distance C and the areas S1 and S2 are set so that "good " range or the most preferable range can be obtained, the fuel-bridge can be prevented from forming at the second discharge gaps. Moreover, since the above distance C is set to be equal to or longer than 0.3mm or equal to or shorter than 1.2 mm, good ignition performance at the second discharge gaps can be maintained.

[0044] The "good" range can be provided even if the ratio S2/S1 is 0, or the confronting domain 901 does not exist. In other words, it is also possible that the free end of second earth electrodes 5 and 6 and the side surfaces of edge portion of insulator 2 do not overlap each other (do not normally oppose to each other), as shown in Fig. 6A.

[0045] Next, the above test on the fuel holding force was conducted while the first discharge gap A and the diameter of the center electrode (the diameter of the edge portion of the center electrode) were changed. The holding force of the first discharge gap will be described hereafter. In the test of the fuel holding force at the second discharge gaps, the distance C and the ratio S2/S1 are set so that the dropping height H can be equal to or longer than 3 cm. In addition, the first discharge gap A and the diameter of the center electrode 3 are variously changed.

[0046] In this example, the size of first earth electrode 4 is 1.4 × 2.6 mm. This size corresponds to 1.4 mm in thickness D and to 2.6 mm in width E of first earth electrode 4 shown in Fig. 4A. The first discharge gap A is changed variously between 0.7 mm and 1.3 mm. If the first discharge gap A is shorter than 0.7 mm, the idle operation of the engine may become rough. On the other hand, if the first discharge gap A is longer than 1.3 mm, the engine misfire may be caused frequently. In addition, the diameter B of center electrode 3 is set between 0.3 mm and 2.8 mm, in view of practical use.

[0047] If the height H is equal to 4cm or less, the fuel-bridge may not form. Therefore, a variety of spark plugs 100 having different gap A and different diameter B of center electrode 3 were prepared and the fuel holding force across the first discharge gap thereof at the height H being 4cm or less was tested. Fig. 8 shows the test result, which indicates relationship between the first discharge gap A and the diameter B of the center electrode 3.

[0048] In a graph shown in Fig. 8, the x-axis indicates the first discharge gap A (mm), and the x-axis indicates the diameter B (mm) of center electrode 3. "Good" spark plugs respectively have the first air gaps A and diameters B in the range including solid line R1 and the right side of the solid line (hereinafter referred to as the good range of first discharge gap). On the other hand, those of "No-good" spark plugs are plotted in the range at the left side of solid line R1. Accordingly, if the first discharge gap A and the diameter B are set so that B ≦ 5A - 2.5 (mm), the fuel-bridge can be prevented from forming at the first discharge gap A. In addition, because first discharge gap A is set between 0.7 mm and 1.3 mm, sufficient ignition performance at the first discharge gap A can be maintained.

[0049] As a comparative example, the fuel holding force of a common spark plug, which has the first earth electrode and no second earth electrode, is measured. This result is shown by broken line R2 in Fig. 8. It has been found that a common spark plug, which the narrower discharge gap A narrower than the double-earth-electrode type spark plug, can prevent the fuel-bridge from forming. Therefore, it is clear that the double-earth-electrode type spark plug according to the present embodiment of the invention can effectively prevent the fuel-bridge from forming.

[0050] According to the above example, the free end of second earth electrodes 5 and 6 and the side surfaces of edge portion 2b of insulator 2 are parallel to each other. However, as shown in Fig. 9 (the first earth electrode is omitted from the drawing), a variation in which the free ends of the second earth electrodes incline within 30° in an angle θ (between the side surface of edge portion 2b of insulator 2 and the free ends of second earth electrodes 5 and 6) is also applicable. If the angle θ is larger than 30°, edge K10 formed at the free ends of second earth electrodes 5 and 6 may project into the discharge gap. This increases wear of the earth electrodes. It is also possible that the angle θ of one earth electrode is different from the other.

[0051] As described above, double-earth-electrode type spark plug according to the present embodiment has been made by setting respective dimensions to be optimum by the specific test method, which has not been noticed as means for preventing the fuel-bridge.

(Other embodiments)

[0052] In the above embodiments, it is possible to provide a tip made of precious metal such as Pt, Ir, or an alloy of the former on the edge of the center electrode and on a portion of the earth electrode opposite thereto. In such a case, the tip corresponds to a portion of the electrodes. It is not always necessary to provide edge portion 2b at the end of insulator.

[0053] The number of the second earth electrodes can be one or three or more. If there are a plurality of earth electrodes and the second discharge gaps, it is not necessary that all the second earth electrodes have the same dimensions C, S1, and S2. The dimensions of each second electrode can be different as far as they are disposed in the same "good" range.

[0054] A spark plug for an internal combustion engine is comprised of a center electrode (3), a cylindrical insulator (2) holding the center electrode in a center hole thereof, a cylindrical metal housing (1) holding the insulator therein, a first earth electrode (4) having a free end disposed opposite the center electrode's edge to form a first discharge gap A, a second earth electrode (5, 6) having a tree end disposed opposite the center electrode's side-surface to form a second discharge gap. The free end of the second earth electrode has a cross-sectional area S1 at a portion normally opposite to the insulator's side surface. The insulator's side surface has a projected area S2 to which the free end of the second earth electrode is normally projected. The free end of the second earth electrode is disposed at a portion outside the outside diameter B of the insulator's edge. There is the following relationship between the first discharge gap A and the outside diameter B: B . 5A - 2.5 (mm).

Claims

1. A spark plug for an internal combustion engine comprising:

a center electrode (3) having a center electrode's edge (3b) and a center electrode's side-surface;

a cylindrical insulator (2) having a center hole and an insulator's edge (2b), said insulator holding said center electrode in said center hole, said insulator's edge having an insulator's side surface;

a cylindrical metal housing (1) holding said insulator therein;

a first earth electrode (4) having a fixed end fixed to said housing and a free end disposed opposite said center electrode's edge to form a first discharge gap A between the same and said center electrode's edge;

a second earth electrode (5, 6) having a fixed end fixed to said metal housing and a free end disposed opposite said center electrode's side-surface to form a second discharge gap between the same and said center electrode's side-surface, said free end having a cross-sectional area S1 normally opposite to said insulator's side surface; characterized in that

said insulator's side surface has a projected area S2 to which said free end of said second earth electrode is normally projected,

said free end of said second earth electrode is disposed at a portion outside the outside diameter B of said insulator's edge,

there is the following relationship between said first discharge gap A and said outside diameter B of said insulator's edge: B . 5A - 2.5 (mm); and

distance C (mm) between said free end of said second earth electrode and said insulator's side surface and a ratio of said projected area S2 to said cross-sectional area S1 are set within a range defined by lines connecting the following points when said distance C is plotted on the x-axis and said ratio is plotted on the y-axis:

(C, S2/S1) = (0.3, 0)

(C, S2/S1) = (1.2, 0)

(C, S2/S1) = (1.2, 1.0)

(C, S2/S1) = (0.8, 1.0)

(C, S2/S1) = (0.3, 0.5)

2. The spark plug according to claim 1, characterized in that

said distance C and said ratio S2/S1 are set within a range defined by lines connecting the following points:

(C, S2/S1) = (0.3, 0)

(C, S2/S1) = (1.2, 0)

(C, S2/S1) = (1.2, 1.0)

(C, S2/S1) = (1.0, 1.0)

(C, S2/S1) = (0.3, 0.3)

3. A spark plug for an internal combustion engine comprising:

a center electrode (3) having a center edge (3b) with an outside diameter B and a center electrode's side surface;

an insulator (2) having a cylindrical insulator's edge (2b) and an insulator's side surface, said insulator covering said center electrode and holding the same;

a metal housing (1) holding said insulator;

a first earth electrode (4) having a fixed end fixed to said housing and a free end disposed opposite said center edge to form a first discharge gap A between the same and said center edge;

a second earth electrode (5, 6) having a fixed end fixed to said metal housing and a free end disposed opposite said center electrode's side surface to form a second discharge gap between the same and said side surface, said free end being disposed at a distance C outside from said insulator's side surface and having a cross-sectional area S1;
characterized in that

said insulator's side surface has a projected area S2 to which a portion of said free end of said second earth electrode is normally projected, and

said first discharge gap A, said outside diameter B, said cross-sectional area S1, said projected area S2, and said distance C are respectively determined so that a fuel bridge is not formed at said first and second discharge gaps when the following test is conducted:

placing said spark plug under temperature of about -25°;,

dipping said spark plug in fuel of an internal combustion engine;

subsequently naturally dropping said spark plug by a prescribed distance with the side of said center edge being down;

holding said spark plug in the air.

4. The spark plug for an internal combustion engine according to claim 3, characterized in that

said prescribed distance is equal to or less than 4 cm.

5. The spark plug for an internal combustion engine according to claim 3, characterized in that

said distance C is between 0.3 mm and 1.2 mm.

6. The spark plug for an internal combustion engine according to claim 1, wherein said first discharge gap A is between 0.7 mm and 1.3 mm.

7. The spark plug for an internal combustion engine according to claim 1, wherein said outside diameter B is between 0.3 mm and 2.8 mm.

Drawing