JP7707985B2 - Spark plugs for internal combustion engines - Google Patents

Description

The present invention relates to a spark plug for an internal combustion engine.

For example, as disclosed in Patent Document 1, a spark plug for an internal combustion engine having a pre-combustion chamber at its tip is known. In this spark plug, the plug cover that covers the pre-combustion chamber has a thin tip. This prevents the tip from becoming too hot, and is intended to prevent the air-fuel mixture from igniting before the spark plug generates a discharge (i.e., pre-ignition).

JP 2020-009747 A

However, while the spark plug described in Patent Document 1 takes into consideration the prevention of overheating of the plug cover, it does not take into consideration the prevention of overheating of the ground electrode. Therefore, there are concerns about pre-ignition originating from the ground electrode, and it can be said that there is room for further improvement.

The present invention was made in consideration of these problems, and aims to provide a spark plug for an internal combustion engine that can prevent overheating of the ground electrode.

The first aspect of the present invention is a cylindrical insulator (3),
a center electrode (4) held on the inner circumferential side of the insulator and exposed on the tip side from the insulator;
a cylindrical housing (2) that holds the insulator on its inner periphery;
a ground electrode (6) forming a discharge gap (G) between itself and the center electrode;
a plug cover (5) provided at the tip of the housing so as to cover a sub-combustion chamber (50) in which the discharge gap is disposed,
The plug cover is formed with a nozzle hole (51) that communicates the auxiliary combustion chamber with the outside,
The ground electrode protrudes into the auxiliary combustion chamber from a fixed end portion (61) fixed to a housing tip side surface (21) of the tip portion of the housing,
the ground electrode has the fixed end and includes an erected portion (62) erected from a front end side surface of the housing toward the front end, a bent portion (63) bent from a front end of the erected portion toward an inner side in a plug radial direction, and an extended portion (64) extended from the bent portion toward an inner side in the plug radial direction,
The ground electrode has an outer layer (65) mainly composed of nickel and a core (66) arranged inside the outer layer and mainly composed of copper,
the discharge gap is formed by a tip end portion of the center electrode and the extension portion of the ground electrode facing each other in an axial direction (Z),
When a height of the ground electrode in the plug axial direction is H mm and a mass ratio of the core portion to the entire ground electrode is Wcu mass%, the following formula (1) is satisfied :
The plug cover has a peripheral wall portion (52) covering an outer periphery side of the auxiliary combustion chamber, a bottom wall portion (53) covering a tip side of the auxiliary combustion chamber, and a corner portion (54) connecting a tip of the peripheral wall portion and an outer periphery of the bottom wall portion in a curved shape,
A gap (g1) is formed between the extension portion and the bottom wall portion, and a distance (D1) between the extension portion and the bottom wall portion in the plug axial direction is equal to or less than a thickness (T2) of the plug cover, in a spark plug (1) for an internal combustion engine.
H≦(2×Wcu/17)+2.5 (1)
The second aspect of the present invention is a cylindrical insulator (3),
a center electrode (4) held on the inner circumferential side of the insulator and exposed on the tip side from the insulator;
a cylindrical housing (2) that holds the insulator on its inner periphery;
a ground electrode (6) forming a discharge gap (G) between itself and the center electrode;
a plug cover (5) provided at the tip of the housing so as to cover a sub-combustion chamber (50) in which the discharge gap is disposed,
The plug cover is formed with a nozzle hole (51) that communicates the auxiliary combustion chamber with the outside,
The ground electrode protrudes into the auxiliary combustion chamber from a fixed end portion (61) fixed to a housing tip side surface (21) of the tip portion of the housing,
the ground electrode has the fixed end and includes an erected portion (62) erected from a front end side surface of the housing toward the front end, a bent portion (63) bent from a front end of the erected portion toward an inner side in a plug radial direction, and an extended portion (64) extended from the bent portion toward an inner side in the plug radial direction,
The ground electrode has an outer layer (65) mainly composed of nickel and a core (66) arranged inside the outer layer and mainly composed of copper,
the discharge gap is formed by a tip end portion of the center electrode and the extension portion of the ground electrode facing each other in an axial direction (Z),
When a height of the ground electrode in the plug axial direction is H mm and a mass ratio of the core portion to the entire ground electrode is Wcu mass%, the following formula (1) is satisfied:
The plug cover has a peripheral wall portion (52) covering an outer periphery side of the auxiliary combustion chamber, a bottom wall portion (53) covering a tip side of the auxiliary combustion chamber, and a corner portion (54) connecting a tip of the peripheral wall portion and an outer periphery of the bottom wall portion in a curved shape,
The plug cover has a cover recess (56) formed by recessing a circumferential part of its inner circumferential surface (55) outward in the plug radial direction,
In the plug cover, the cover recess is open to a base end side,
The cover recess is formed from the peripheral wall portion to the corner portion,
At least a portion of the erected portion is disposed inside the cover recess.
H≦(2×Wcu/17)+2.5 (1)

The spark plug has a ground electrode with a core mainly composed of copper and satisfies the above formula (1). Therefore, heat from the ground electrode can be easily dissipated to the outside, and the ground electrode can be prevented from receiving heat from the combustion in the main combustion chamber. As a result, overheating of the ground electrode can be prevented.

As described above, according to the above aspect, it is possible to provide a spark plug for an internal combustion engine that is capable of suppressing overheating of the ground electrode.
In addition, the symbols in parentheses described in the claims and the means for solving the problems indicate a correspondence with the specific means described in the embodiments described below, and do not limit the technical scope of the present invention.

3 is a cross-sectional view taken along line II in FIG. 2 , showing a portion near the tip of the spark plug according to the first embodiment; FIG. FIG. 2 is a cross-sectional view taken along line II-II of FIG. 4 is a cross-sectional view showing the height H of a ground electrode in the first embodiment. FIG. 4 is a cross-sectional view perpendicular to the extending direction of the ground electrode in the first embodiment, taken along line IV-IV in FIG. 2 . 1 is a cross-sectional view of an internal combustion engine in which a spark plug is installed according to a first embodiment. FIG. 4 is a diagram showing a state in which an electrode material is joined to a housing in the first embodiment. FIG. 4 is a diagram showing a state in which the electrode material is bent by a bending jig in the first embodiment. FIG. 4 is a diagram showing a state in which the electrode material is bent by a hand press in the first embodiment. FIG. 4 is a cross-sectional view of a first member in the first embodiment. FIG. 4 is a cross-sectional view showing a state in which the clad member is assembled to the first member in the first embodiment. FIG. 4 is a cross-sectional view showing a state in which the clad member is pressed by a pressing jig in the first embodiment. 13 is a graph showing the relationship between height H and the advance angle from MBT in Experimental Example 1. 13 is a graph showing the relationship between mass ratio Wcu and height H in Experimental Example 1.

(Embodiment 1)
An embodiment of a spark plug for an internal combustion engine will be described with reference to FIGS.
1 and 2, a spark plug 1 for an internal combustion engine according to this embodiment has a cylindrical insulator 3, a center electrode 4, a cylindrical housing 2, a ground electrode 6, and a plug cover 5. The center electrode 4 is held on the inner periphery of the insulator 3 and is exposed from the insulator 3 at the tip end. The housing 2 holds the insulator 3 on the inner periphery. The ground electrode 6 forms a discharge gap G between itself and the center electrode 4. The plug cover 5 is provided at the tip of the housing 2 so as to cover the auxiliary combustion chamber 50 in which the discharge gap G is located. The plug cover 5 has an injection hole 51 formed therein, which communicates the auxiliary combustion chamber 50 with the outside.

The ground electrode 6 protrudes into the auxiliary combustion chamber 50 from a fixed end 61 fixed to the housing tip side surface 21 at the tip of the housing 2. The ground electrode 6 has an erect portion 62, a bent portion 63, and an extended portion 64. The erect portion 62 has the fixed end 61 and erects from the housing tip side surface 21 toward the tip side. The bent portion 63 bends from the tip of the erect portion 62 toward the inside in the plug radial direction. The extended portion 64 extends from the bent portion 63 toward the inside in the plug radial direction.

The ground electrode 6 also has an outer layer 65 whose main component is nickel, and a core 66 disposed inside the outer layer 65 and whose main component is copper.

The discharge gap G is formed by the tip of the center electrode 4 and the extension 64 of the ground electrode 6 facing each other in the plug axial direction Z.

3, the height of the ground electrode 6 in the plug axial direction Z is H mm, and the mass ratio of the core portion 66 to the entire ground electrode 6 is Wcu mass %. In this case, the spark plug 1 satisfies the following formula (1).
H≦(2×Wcu/17)+2.5 (1)

It is more preferable that the spark plug 1 further satisfies the following formula (2).
H≦(2×Wcu/17)+0.5 (2)

The spark plug 1 of this embodiment can be used as an ignition means in an internal combustion engine of an automobile or the like. As shown in FIG. 5, the threaded portion 22 of the housing 2 is screwed into the female threaded portion of the plug hole 711 of the cylinder head 71, and the spark plug 1 is attached to the internal combustion engine 10. The housing 2 is in thermal contact with the cylinder head 71.

The internal combustion engine 10 has a piston 74 that reciprocates within a cylinder 70. The volume of the main combustion chamber 101 changes due to the reciprocating motion of the piston 74. The internal combustion engine 10 has an intake port 721 and an exhaust port 731, each of which is provided with an intake valve 72 or an exhaust valve 73.

One end of the spark plug 1 in the axial direction Z is disposed in the main combustion chamber 101 of the internal combustion engine 10. In the axial direction Z of the spark plug 1, the side exposed to the main combustion chamber 101 is referred to as the tip side, and the opposite side is referred to as the base side. The axial direction Z of the spark plug 1 is also referred to as the plug axial direction Z or simply the Z direction, as appropriate. The plug central axis C refers to the central axis C of the spark plug 1. The plug radial direction refers to the radial direction of a circle centered on the plug central axis C on a plane perpendicular to the plug central axis C. In this embodiment, the plug central axis C is also the central axis of the center electrode 4.

In this embodiment, the plug cover 5 is joined to the tip of the housing 2 by welding or the like. When the spark plug 1 is attached to the internal combustion engine 10, the plug cover 5 separates the auxiliary combustion chamber 50 from the main combustion chamber 101. The injection hole 51 also connects the auxiliary combustion chamber 50 and the main combustion chamber 101.

As shown in FIG. 3, the plug cover 5 has a peripheral wall portion 52, a bottom wall portion 53, and a corner portion 54. The peripheral wall portion 52 is a generally cylindrical portion that covers part of the outer periphery of the auxiliary combustion chamber 50. The bottom wall portion 53 is a portion that covers the tip side of the auxiliary combustion chamber 50. The corner portion 54 is a portion that connects the tip of the peripheral wall portion 52 and the outer periphery of the bottom wall portion 53 in a curved shape. The base end portion of the peripheral wall portion 52 is joined to the tip portion of the housing 2. The plug cover 5 is in thermal contact with the housing 2.

The nozzle hole 51 is formed in the corner 54 of the plug cover 5. The nozzle hole 51 opens at an angle in the Z direction so that it moves outward in the plug radial direction toward the tip side.

As shown in Figs. 1 and 2, the plug cover 5 has a cover recess 56 formed by recessing a portion of the inner peripheral surface 55 in the circumferential direction outward in the plug radial direction. The cover recess 56 is formed from the peripheral wall portion 52 to the corner portion 54. The cover recess 56 opens on the base end side. At least a portion of the erected portion 62 of the ground electrode 6 is disposed inside the cover recess 56.

In this embodiment, the ground electrode 6 is fixed to the housing tip side surface 21 without abutting the plug cover 5. The ground electrode 6 is in thermal contact with the housing 2. As shown in FIG. 3, a gap g1 is formed between the extension portion 64 of the ground electrode 6 and the bottom wall portion 53 of the plug cover 5. The distance D1 between the extension portion 64 and the bottom wall portion 53 in the Z direction is equal to or less than the thickness T2 of the plug cover 5. In this embodiment, the distance D1 is 0.5 to 1.0 mm. The height H of the ground electrode 6 in the Z direction is equal to or greater than 1.5 mm.

As shown in FIG. 4, the cross section perpendicular to the extension direction of the ground electrode 6 has a substantially rectangular parallelepiped shape. The thickness T1 of the ground electrode 6 is smaller than the width W1 of the ground electrode 6. In this embodiment, the thickness T1 is 1.0 mm or more.

As shown in FIG. 3, the fixed end 61 of the ground electrode 6 is joined to the housing tip side surface 21 by welding. The outer layer portion 65 and the core portion 66 are each directly fixed to the housing tip side surface 21. The core portion 66 is sealed by the outer layer portion 65 and the housing tip side surface 21.

The outer layer 65 is formed from the fixed end 61 to the protruding end 67 of the ground electrode 6. The core 66 is formed from the fixed end 61 to the extension 64. In this embodiment, the core 66 is not provided at the protruding end 67.

In this embodiment, the mass ratio Wcu of the core 66 to the entire ground electrode 6 is 10 mass% or more and 25 mass% or less. The core 66 has a higher thermal conductivity than the outer layer 65. In this embodiment, the copper content in the core 66 is 99 mass% or more. The nickel content in the outer layer 65 is 85 mass% or more.

The ground electrode 6 is disposed inside the core 66 and has a shaft 68 that is mainly composed of nickel. The shaft 68 is formed from the fixed end 61 to the extension 64. The nickel content in the shaft 68 is 85% by mass or more.

As shown in Figures 1, 3, and 4, the ground electrode 6 has a multi-layer structure in which an outer layer 65, a core 66, and an axis 68 are layered in both the thickness direction and width direction of the ground electrode 6.

As shown in Figs. 1 and 3, a tip 69 is joined to the protruding end 67 of the extension 64. A tip 41 is also joined to the tip of the center electrode 4. The discharge gap G is formed by the flat base end surface of the tip 69 and the flat tip surface of the tip 41 facing each other in the Z direction. As shown in Fig. 2, the core 66 is formed so as not to overlap the tip 69 when viewed from the Z direction. The tips 41 and 69 can be made of, for example, a precious metal such as iridium or platinum, or an alloy containing these as the main component.

Also, as shown in Figures 1 and 3, the discharge gap G is formed on the tip side of the tip of the housing 2. In other words, the tip of the center electrode 4 is located on the tip side of the tip of the housing 2.

Next, a method for forming the ground electrode 6 in this embodiment will be described.
In this embodiment, the ground electrode 6 is formed by bending a rod-shaped electrode material. Specifically, first, as shown in FIG. 6, a substantially rectangular prism-shaped electrode material 60 is joined to the housing front end side surface 21 of the housing 2 before the center electrode 4, the insulator 3, and the plug cover 5 are assembled. When joining the electrode material 60, the base end surface of the electrode material 60 and the housing front end side surface 21 are abutted against each other in the Z direction while the longitudinal direction of the electrode material 60 is aligned along the Z direction, and welding is performed. Then, a tip 69 is welded to the electrode material 60. In addition, the length of the electrode material 60 in the Z direction is adjusted by cutting a part of the electrode material 60 as necessary.

Then, insert the insertion jig 91 into the housing 2 to which the electrode material 60 is joined. At this time, the insertion jig 91 is positioned so that the tip of the insertion jig 91 is located further toward the tip side than the tip of the housing 2. In addition, the amount by which the insertion jig 91 protrudes from the tip of the housing 2 toward the tip side is adjusted according to the height H of the ground electrode 6.

Then, as shown in FIG. 7, the electrode material 60 joined to the housing 2 is bent inward in the plug radial direction using a bending jig 92. Specifically, the bending jig 92 has a recess 921 for inserting a part of the electrode material 60. Then, with a part of the electrode material 60 inserted into this recess 921, the electrode material 60 is bent inward in the plug radial direction.

Then, as shown in FIG. 8, a portion of the bent electrode material 60 is pressed toward the base end by a hand press 93. This forms a ground electrode 6 having an erect portion 62, a bent portion 63, and an extended portion 64, as shown in FIG. 1 and FIG. 3.

After the ground electrode 6 is formed, the insertion jig 91 is removed, and the insulator 3, which holds the center electrode 4 on its inner circumference, is assembled into the housing 2. Thereafter, if necessary, the ground electrode 6 fixed to the housing 2 is slightly deformed to fine-tune the discharge gap G to an appropriate distance. The plug cover 5 is then joined to the tip of the housing 2. This allows the spark plug 1 of this embodiment to be manufactured, as shown in Figures 1 to 3.

Next, a method for manufacturing the electrode member 60 to be joined to the housing 2 will be described.
In manufacturing the electrode material 60, first, a substantially cylindrical member mainly composed of nickel is pressed to produce a bottomed cylindrical first member 650 which becomes the outer layer portion 65, as shown in FIG. 9. Then, as shown in FIG. 10, a substantially cylindrical clad member 601 is assembled to the inside of a recess 651 of the first member 650. The clad member 601 has a cylindrical second member 660 mainly composed of copper which becomes the core portion 66, and a third member 680 mainly composed of nickel which becomes the shaft portion 68. The third member 680 is disposed inside the second member 660. In the clad member 601, the second member 660 and the third member 680 are pressure-bonded to each other.

Next, as shown in FIG. 11, the clad member 601 assembled to the first member 650 is pressed by a pressing jig 94 to press the clad member 601 and the first member 650 together. After that, the member consisting of the clad member 601 and the first member 650 is annealed. Next, the annealed member is formed into an approximately rectangular prism shape using a mold. After that, the member that has become approximately rectangular prism shape is further annealed. Then, unnecessary parts are removed from the annealed member as necessary, whereby the electrode material 60 can be produced.

Next, the effects of this embodiment will be described.
The spark plug 1 has a ground electrode 6 with a core 66 mainly made of copper, and satisfies the above formula (1). Therefore, heat from the ground electrode 6 can be easily dissipated to the outside, and the ground electrode 6 can be prevented from receiving heat due to combustion in the main combustion chamber 101. As a result, overheating of the ground electrode 6 can be prevented.

The spark plug 1 of this embodiment ignites the mixture in the auxiliary combustion chamber 50 by generating a discharge in the discharge gap G, forming a flame. The flame generated in the auxiliary combustion chamber 50 is then ejected as a flame jet into the main combustion chamber 101 through the nozzle hole 51. This causes the flame to propagate to the main combustion chamber 101 and burn the mixture. Here, the temperature of the main combustion chamber 101 during combustion is likely to be highest near the center of the combustion, that is, the center of the main combustion chamber 101, due to the cooling effect of the cylinder 70 and the piston 74. Therefore, the lower the height H and the smaller the amount of protrusion of the ground electrode 6 into the main combustion chamber 101, the easier it is to suppress the heat reception of the ground electrode 6. In addition, the lower the height H, the shorter the heat dissipation path from the protruding end 67, which is particularly likely to become hot, to the housing tip side surface 21, making it easier to suppress overheating of the ground electrode 6. In addition, the ground electrode 6 has a core 66 mainly composed of copper, which has a relatively high thermal conductivity. Therefore, the higher the mass ratio Wcu, the easier it is to dissipate heat from the ground electrode 6 to the outside through the housing 2, and the easier it is to suppress overheating of the ground electrode 6. On the other hand, if the height H is too small, when bending the electrode material 60, a load is likely to be applied to the joint between the housing 2 and the ground electrode 6, and manufacturability is likely to be poor. Also, if the mass ratio Wcu is too high, it becomes difficult to sufficiently seal the core portion 66 by the outer layer portion 65, and manufacturability of the ground electrode 6 is likely to be poor. Therefore, the spark plug 1 of this embodiment satisfies the above formula (1). By adjusting the height H and the mass ratio Wcu within the range of the above formula (1), it is possible to suppress overheating of the ground electrode 6 while ensuring manufacturability of the ground electrode 6. Therefore, pre-ignition can be suppressed during high loads when the operating efficiency of the internal combustion engine is high. As a result, the output and fuel efficiency of the internal combustion engine can be improved.

As described above, the spark plug 1 can easily dissipate heat from the ground electrode 6 to the outside and can suppress heat reception by the ground electrode 6, so that even if heat from the bottom wall portion 53 is transferred to the extension portion 64 through the gap g1 (see FIG. 3), overheating of the ground electrode 6 can be sufficiently suppressed. In particular, if the gap g1 is small, the ground electrode 6 is likely to receive a large amount of heat from the bottom wall portion 53 of the plug cover 5, but the configuration of this embodiment can suppress overheating of the ground electrode 6.

In addition, by reducing the height H, the height of the plug cover 5 in the Z direction can also be reduced. In other words, the amount of protrusion of the plug cover 5 into the main combustion chamber can be reduced. This makes it possible to lengthen the distance from the center of the main combustion chamber 101 to the plug cover 5, and further shorten the heat dissipation path from the tip of the plug cover 5 to the housing 2. As a result, overheating of the plug cover 5 can be suppressed.

Furthermore, if the spark plug 1 further satisfies the above formula (2), it becomes easier to dissipate heat from the ground electrode 6 to the outside, and heat reception by the ground electrode 6 can be further suppressed. Therefore, overheating of the ground electrode 6 can be further suppressed. As a result, even if the spark plug 1 is used for a high-efficiency engine such as an automobile engine with a high compression ratio or an automobile engine equipped with a turbocharger, pre-ignition can be sufficiently suppressed.

The height H is 1.5 mm or more. Therefore, it is easy to form the ground electrode 6 having the standing portion 62, the bent portion 63, and the extended portion 64. As a result, the productivity of the spark plug 1 can be improved.

The core 66 is also fixed directly to the housing tip side surface 21. This allows the heat of the ground electrode 6 to be efficiently transferred to the housing 2. As a result, overheating of the ground electrode 6 can be further suppressed.

In addition, the protruding end 67 does not have a core 66. Therefore, when welding the tip 69 to the electrode material 60, it is less affected by the core 66, which is mainly composed of copper. Therefore, it is easy to weld the tip 69 to the electrode material 60. As a result, the productivity of the spark plug 1 can be improved.

The discharge gap G is formed closer to the tip than the tip of the housing 2. Therefore, it is easy to check the distance of the discharge gap G formed between the ground electrode 6 and center electrode 4 fixed to the housing 2 before fixing the plug cover 5 to the housing 2. This makes it easy to adjust the discharge gap G. As a result, the productivity of the spark plug 1 can be improved.

In addition, at least a portion of the erected portion 62 is disposed inside the cover recess 56. This makes it easier to widen the joint area between the ground electrode 6 and the housing tip side surface 21. In other words, since a portion of the erected portion 62 is disposed inside the cover recess 56, the joint area between the ground electrode 6 and the housing tip side surface 21 can be widened. This makes it easier for heat from the ground electrode 6 to transfer to the housing 2. As a result, overheating of the ground electrode 6 can be further suppressed.

The ground electrode 6 has an erected portion 62, a bent portion 63, and an extended portion 64. This ensures the strength of the joint between the ground electrode 6 and the housing 2, and further suppresses overheating of the ground electrode 6. In other words, in this embodiment, the base end surface of the rod-shaped electrode material 60 and the housing tip side surface 21 are abutted against each other in the Z direction and welded together, thereby fixing the ground electrode 6 to the housing 2. This allows the ground electrode 6 to be firmly joined to the housing 2. In addition, the heat of the ground electrode 6 can be dissipated to the outside more efficiently via the joint between the housing 2 and the ground electrode 6.

The core 66 is sealed by the outer layer 65 and the housing tip side surface 21. This ensures that the core 66, which is mainly composed of copper, which has a relatively low melting point, is prevented from dissolving. As a result, the heat dissipation properties of the ground electrode 6 can be reliably maintained.

As described above, this embodiment provides a spark plug 1 for an internal combustion engine that can prevent the ground electrode 6 from overheating.

In the above embodiment 1, the ground electrode 6 has a shaft portion 68. However, the ground electrode may also be configured not to have a shaft portion.

(Experimental Example 1)
In this example, a plurality of spark plugs having the same basic structure as in the first embodiment but different values of height H and mass ratio Wcu were used to analyze the relationship between the value of height H and mass ratio Wcu and the advance amount from MBT (abbreviation of Minimum Spark Advance for Best Torque) when pre-ignition occurs. Generally, the more the ignition timing is advanced from MBT, the more likely pre-ignition occurs. Therefore, in this example, for the internal combustion engines equipped with each spark plug, the ignition timing was gradually advanced from MBT, and the ignition timing at which pre-ignition occurs was analyzed. The test conditions were a single-cylinder four-stroke engine with the throttle fully open and the rotation speed of 6500 rpm.

Here, the criterion for sufficient suppression of pre-ignition when used as a spark plug for a general automobile engine is when the ignition timing at which pre-ignition occurs is advanced by 5° CA (abbreviation of crank angle) or more from MBT. From the above analysis results, the relationship between the height H and mass ratio Wcu that satisfies this criterion was determined. Also, the criterion for sufficient suppression of pre-ignition when used as a spark plug for a high-efficiency engine such as an automobile engine with a high compression ratio or an automobile engine equipped with a supercharger is when the ignition timing at which pre-ignition occurs is advanced by 10° CA or more from MBT. From the above analysis results, the relationship between the height H and mass ratio Wcu that satisfies this criterion was determined. Also, in this example, MBT is 20° CA before BTDC (abbreviation of top dead center of compression).

The graph in Figure 12 shows the relationship between height H and the amount of advance from MBT for spark plugs with different mass fractions Wcu. In the graph in Figure 12, the circles and squares represent plots of the analysis results for each spark plug. The graph in Figure 12 also shows approximation lines for these plots.

From the graph in Figure 12, it can be seen that the smaller the height H, the more advanced the ignition timing at which pre-ignition occurs, regardless of the value of the mass ratio Wcu. From this result, it is believed that the shorter the height H, the more the plug cover can be prevented from overheating, and pre-ignition can be prevented.

The graph in Figure 12 also shows that, regardless of the value of height H, the greater the value of mass ratio Wcu, the more advanced the ignition timing at which pre-ignition occurs. From these results, it can be considered that the greater the value of mass ratio Wcu, the more the plug cover can be prevented from overheating, and the more pre-ignition can be prevented.

Further, from each of the approximation lines shown in the graph of FIG. 12, the height H when the advance amount from the MBT is 5° CA and the height H when the advance amount from the MBT is 10° CA were obtained. The graph of FIG. 13 shows the relationship between the obtained height H and the mass ratio Wcu. In the graph of FIG. 13, the circles are plots of the values of the height H obtained from the approximation lines of the graph of FIG. 12. The graph of FIG. 13 also shows the approximation lines in these plots. Here, the equation of the approximation line when the advance amount from the MBT is 5° CA is the following equation (3). The equation of the approximation line when the advance amount from the MBT is 10° CA is the following equation (4).
H=(2×Wcu/17)+2.5...(3)
H=(2×Wcu/17)+0.5...(4)

In the graph of FIG. 13, the range below the approximate straight line of formula (3) is the combination of height H and mass ratio Wcu at which the ignition timing at which pre-ignition occurs is advanced from MBT by 5° CA or more. In other words, the spark plug of embodiment 1 that satisfies the above formula (1) can sufficiently suppress pre-ignition when used as a spark plug for a general automobile engine. In the graph of FIG. 13, the range below the approximate straight line of formula (4) is the combination of height H and mass ratio Wcu at which the ignition timing at which pre-ignition occurs is advanced from MBT by 10° CA or more. In other words, the spark plug that satisfies the above formula (2) can sufficiently suppress pre-ignition when used as a spark plug for a high-efficiency engine.

(Experimental Example 2)
In this example, as shown in Table 1 below, a relationship between the height H and the manufacturability of the ground electrode was analyzed using a plurality of spark plugs with different heights H. In each spark plug, the electrode material for the ground electrode had a width of 2.15 mm and a thickness of 1.0 mm. Sample 1 is a spark plug having the same basic structure as that of the first embodiment, but with a ground electrode mass ratio Wcu of 17 mass%. That is, Sample 1 has a core portion mainly composed of copper. Samples 2 to 5 are spark plugs having a ground electrode without a core portion and an axial portion. Other configurations of Samples 2 to 5 are the same as those of the first embodiment.

In this example, the manufacturability was evaluated based on the bending strength when bending the ground electrode and the strength of the joint between the fixed end of the ground electrode and the housing tip side surface. The bending strength was evaluated as shown in Table 1, with "○" if no cracks were found in the bent portion and "×" if cracks were found in the bent portion. The joint strength was evaluated as shown in Table 1, with "○" if no peeling was found in the joint between the fixed end of the ground electrode and the housing tip side surface and "×" if peeling was found in the joint. The manufacturability judgment shown in Table 1 was "○" if both bending strength and joint strength were evaluated as "○", and "×" if at least one of the bending strength evaluation and joint strength evaluation was evaluated as "×". If the judgment in Table 1 was "○", it was determined that there was no problem with manufacturability.

As can be seen from Table 1, samples 1 and 3 to 5 were judged as "○" for manufacturability. On the other hand, sample 2 was rated as "×" for joint strength, resulting in a manufacturability judgement of "×". Sample 2 and samples 3 to 5 have the same configuration except for the height H. Therefore, it is believed that sample 2 was rated as "×" for joint strength because the height H was too low, placing too much load on the joint. Furthermore, samples 3 to 5, which have height H of 1.5 mm or more, were judged as "○" for manufacturability. Therefore, it is believed that by making height H 1.5 mm or more, it is possible to reliably ensure the manufacturability of the ground electrode.

In addition, even though sample 1 has a height H of 1.3 mm like sample 2, the joint strength was evaluated as "○" and the manufacturability was judged as "○". Here, sample 1 has a core mainly composed of copper. Therefore, it is presumed that the hardness of the electrode material is relatively low and the electrode material can be easily bent. Therefore, it is presumed that when the electrode material is bent, the joint is not easily loaded, and the joint strength was evaluated as "○". From this result, it is considered that in the case of a ground electrode having a core, the manufacturability of the ground electrode can be reliably ensured by setting the height H of the ground electrode to 1.3 mm or more. In addition, when the height H is 1.5 mm or more, the manufacturability was judged as "○" even for samples 3 to 5 that do not have a core, so it is considered that by having a core and further setting the height H to 1.5 mm or more as in embodiment 1, the manufacturability can be reliably improved.

The present invention is not limited to the above-described embodiments, and can be applied to various embodiments without departing from the spirit of the present invention.

1...spark plug, 2...housing, 21...side of housing tip, 3...insulator, 4...center electrode, 5...plug cover, 50...auxiliary combustion chamber, 51...nozzle hole, 6...ground electrode, 61...fixed end, 62...standing portion, 63...bent portion, 64...extended portion, 65...outer layer portion, 66...core portion, G...discharge gap, Z...plug axial direction

Claims (5)

A cylindrical insulator (3);
a center electrode (4) held on the inner circumferential side of the insulator and exposed on the tip side from the insulator;
a cylindrical housing (2) that holds the insulator on its inner periphery;
a ground electrode (6) forming a discharge gap (G) between itself and the center electrode;
a plug cover (5) provided at the tip of the housing so as to cover a sub-combustion chamber (50) in which the discharge gap is disposed,
The plug cover is formed with a nozzle hole (51) that communicates the auxiliary combustion chamber with the outside,
The ground electrode protrudes into the auxiliary combustion chamber from a fixed end portion (61) fixed to a housing tip side surface (21) of the tip portion of the housing,
the ground electrode has the fixed end and includes an erected portion (62) erected from a front end side surface of the housing toward the front end, a bent portion (63) bent from a front end of the erected portion toward an inner side in a plug radial direction, and an extended portion (64) extended from the bent portion toward an inner side in the plug radial direction,
The ground electrode has an outer layer (65) mainly composed of nickel and a core (66) arranged inside the outer layer and mainly composed of copper,
the discharge gap is formed by a tip end portion of the center electrode and the extension portion of the ground electrode facing each other in an axial direction (Z),
When a height of the ground electrode in the plug axial direction is H mm and a mass ratio of the core portion to the entire ground electrode is Wcu mass%, the following formula (1) is satisfied :
The plug cover has a peripheral wall portion (52) covering an outer periphery side of the auxiliary combustion chamber, a bottom wall portion (53) covering a tip side of the auxiliary combustion chamber, and a corner portion (54) connecting a tip of the peripheral wall portion and an outer periphery of the bottom wall portion in a curved shape,
a gap (g1) is formed between the extension portion and the bottom wall portion, and a distance (D1) between the extension portion and the bottom wall portion in the plug axial direction is equal to or less than a thickness (T2) of the plug cover .
H≦(2×Wcu/17)+2.5 (1) The plug cover has a cover recess (56) formed by recessing a circumferential part of its inner circumferential surface (55) outward in the plug radial direction,
In the plug cover, the cover recess is open to a base end side,
The cover recess is formed from the peripheral wall portion to the corner portion,
2. The spark plug for an internal combustion engine according to claim 1, wherein at least a portion of said upstanding portion is disposed inside said cover recess. A cylindrical insulator (3);
a center electrode (4) held on the inner circumferential side of the insulator and exposed on the tip side from the insulator;
a cylindrical housing (2) that holds the insulator on its inner periphery;
a ground electrode (6) forming a discharge gap (G) between itself and the center electrode;
a plug cover (5) provided at the tip of the housing so as to cover a sub-combustion chamber (50) in which the discharge gap is disposed,
The plug cover is formed with a nozzle hole (51) that communicates the auxiliary combustion chamber with the outside,
The ground electrode protrudes into the auxiliary combustion chamber from a fixed end portion (61) fixed to a housing tip side surface (21) of the tip portion of the housing,
the ground electrode has the fixed end and includes an erected portion (62) erected from a front end side surface of the housing toward the front end, a bent portion (63) bent from a front end of the erected portion toward an inner side in a plug radial direction, and an extended portion (64) extended from the bent portion toward an inner side in the plug radial direction,
The ground electrode has an outer layer (65) mainly composed of nickel and a core (66) arranged inside the outer layer and mainly composed of copper,
the discharge gap is formed by a tip end portion of the center electrode and the extension portion of the ground electrode facing each other in an axial direction (Z),
When a height of the ground electrode in the plug axial direction is H mm and a mass ratio of the core portion to the entire ground electrode is Wcu mass%, the following formula (1) is satisfied :
The plug cover has a peripheral wall portion (52) covering an outer periphery side of the auxiliary combustion chamber, a bottom wall portion (53) covering a tip side of the auxiliary combustion chamber, and a corner portion (54) connecting a tip of the peripheral wall portion and an outer periphery of the bottom wall portion in a curved shape,
The plug cover has a cover recess (56) formed by recessing a circumferential part of its inner circumferential surface (55) outward in the plug radial direction,
In the plug cover, the cover recess is open to a base end side,
The cover recess is formed from the peripheral wall portion to the corner portion,
At least a portion of the erected portion is disposed inside the cover recess .
H≦(2×Wcu/17)+2.5 (1) 4. The spark plug for an internal combustion engine according to claim 1, wherein the height H is 1.5 mm or more. 4. A spark plug for an internal combustion engine according to claim 1 , further satisfying the following formula (2):
H≦(2×Wcu/17)+0.5 (2)