Patent Application
20050264153
This application claims priority from Japanese Patent Application No. 2004-161827, filed on May 31, 2004, the content of which is hereby incorporated by reference into this application.
1. Technical Field of the Invention
The present invention relates generally to spark plugs for internal combustion engines. More particularly, the invention relates to a spark plug which has a center electrode, a first ground electrode aligned with the center electrode in an axial direction of the spark plug to form a first spark gap across which sparks are discharged in normal conditions, and a second ground electrode aligned with the center electrode in a radial direction of the spark plug to form a second spark gap across which sparks are discharged when an insulator of the spark plug is fouled with carbon deposits.
2. Description of the Related Art
Existing spark plugs with a plurality of ground electrodes generally include a tubular metal shell, a cylindrical insulator, a center electrode, a first ground electrode, and a second ground electrode.
The metal shell has a threaded portion for fitting the spark plug into a combustion chamber of an engine.
The insulator has a bore formed therethrough in an axial direction thereof and is fixed in the metal shell such that an end thereof protrudes from an end of the metal shell.
The center electrode is secured in the bore of the insulator and includes a first diameter portion, a second diameter portion, and an intermediate portion provided between the first and second diameter portions. The first diameter portion is positioned outside the bore of the insulator and has a free end. The second diameter portion has a diameter greater than that of the first diameter portion. The intermediate portion has a first interface with the first diameter portion and a second interface with the second diameter portion and tapers from the second interface to the first interface.
The first ground electrode has a base end joined to the end of the metal shell and a tip portion that faces the end of the first diameter portion of the center electrode through a first spark gap in the axial direction of the insulator.
The second electrode has a base end joined to the end of the metal shell and a tip portion that faces the second interface of the intermediate portion of the center electrode with the second diameter portion of the same through a second spark gap in a radial direction of the insulator.
In such a spark plug, normal sparks are discharged across the first spark gap in normal conditions of the spark plug.
However, when the insulator of the spark plug is fouled with carbon deposits, “side sparks” may be discharged, instead of the normal sparks, across the second spark gap.
Specifically, with reference to
When the insulator 2 is fouled with carbon deposits, the insulation properties of the insulator 2 are degraded, or even lost. However, with the above arrangement, the spark plug can clean the insulator 2 by itself through burning off the carbon deposits with the side sparks, thereby recovering the insulation properties of the insulator 2.
Japanese Unexamined Patent Publication No. 2001-93645 and Japanese Patent No. 3272615 disclose spark plugs designed to maximize such a “self-clean” effect by burning off the carbon deposits with side sparks. In those spark plugs, the second interface 3bc of the intermediate portion 3b with the second diameter portion 3c is positioned inside the bore 2b of the insulator 2, and side sparks are discharged along a discharge path Z as shown in
However, in the above spark plugs, since side sparks pass through the whole length between the inner and outer edge of the end 2a of the insulator 2, a channeling problem tends to occur. The channeling problem here denotes a phenomenon in which the end 2a of the insulator 2 is melted due to the heat energy transferred thereto from the side sparks that creep through the end 2a, thus forming channels along the discharge path Z on the end 2a.
On the other hand, Japanese Patent No. 3140006 discloses a spark plug in which the second interface 3bc of the intermediate portion 3b with the second diameter portion 3c is positioned outside the bore 2b of the insulator 2. However, in the spark plug, the discharge path Z, along which side sparks are discharged, still includes the whole length between the inner and outer edge of the end 2a of the insulator 2. Consequently, the channeling problem still tends to occur.
The present invention has been made in view of the above-mentioned problem.
It is, therefore, a primary object of the present invention to provide a spark plug that secures the insulation properties of an insulator thereof by burning off the carbon deposits on the insulator with side sparks, while preventing the channeling problem from occurring.
Through experimental investigation, the inventor of the present invention has found that it is possible to prevent the channeling problem by allowing side sparks to be discharged along a discharge path Z as shown in
The inventor has also found that it is possible to recover the insulation properties of the insulator even if the carbon deposits on the insulator may not be completely burnt off with the side sparks discharged along the discharge path Z as shown in
The present invention is derived from the results of the experimental investigation.
According to the first embodiment of the present invention, a spark plug S1 is provided which includes:
With the above structure, when the insulator of the spark plug S1 is fouled with carbon deposits, side sparks are discharged without formation of channels on the end of the insulator, thereby recovering the insulation properties of the insulator.
According to the second embodiment of the present invention, a spark plug S2 is provided which includes:
With the above structure, when the insulator of the spark plug S2 is fouled with carbon deposits, side sparks are discharged without formation of channels on the end of the insulator, thereby recovering the insulation properties of the insulator
It is preferable to define for the spark plugs S1 and S2 the following dimensional relationship:
0≦F≦0.5H, where
As a result, the insulation properties of the insulator can be recovered when it is fouled with carbon deposits, while suppressing formation of a fuel bridge in the spark plug.
It is preferable to define for the spark plugs S1 and S2 the following dimensional relationship:
E<A,
where E is a size of the first spark gap between the end of the first diameter portion of the center electrode and the tip portion of the first ground electrode in the axial direction of the insulator.
As a result, normal sparks can be reliably discharged across the first spark gap when the insulator of the spark plug is not fouled with carbon deposits.
It is preferable to define for the spark plugs S1 and S2 the dimensional relationship of B<E.
As a result, the ignition capability of the spark plug can be secured with the help of side sparks when the insulator of the spark plug is fouled with carbon deposits.
It is preferable to define for the spark plugs S1 and S2 the following dimensional relationship:
(A−B)<2D,
where D is a width of a cross section of the tip portion of the second ground electrode in a direction perpendicular to the axial direction of the insulator, the cross section being perpendicular to the radial direction of the insulator.
As a result, the ignition capability of the spark plug can be secured with the help of side sparks when the insulator of the spark plug is fouled with carbon deposits.
It is preferable that in the spark plugs S1 and S2, the first diameter portion of the center electrode is made up of a noble metal chip.
It is also preferable that in the spark plugs S1 and S2, the tip portion of the first ground electrode includes a noble metal chip provided thereon such that the noble metal chip faces the end of the first diameter portion of the center electrode in the axial direction of the insulator.
Using a noble metal chip, the space available for ignition in the first spark gap of the spark plug is increased, while the noble metal chip is made not too thin to be easily worn down.
Further, the noble metal chip is preferably made of one of a Pt-based alloy and an Ir-based alloy.
Specifying the material of the noble chip as above, a long service life is secured for the center electrode and/or the first ground electrode of the spark plug.
The present invention will be understood more fully from the detailed description given hereinafter and from the accompanying drawings of the preferred embodiments of the invention, which, however, should not be taken to limit the invention to the specific embodiments but are for the purpose of explanation and understanding only.
In the accompanying drawings:
The preferred embodiments of the present invention will be described hereinafter with reference to
It should be noted that, for the sake of clarity and understanding, identical components having identical functions in different embodiments of the invention have been marked, where possible, with the same reference numerals in each of the figures.
The spark plug S1 is designed for use in internal combustion engines of automotive vehicles. The installation of the spark plug S1 in an internal combustion engine is achieved by fitting it into a combustion chamber (not shown) of the engine through a threaded bore provided in the engine head (not shown).
As shown in
The tubular metal shell 1 is made of a conductive metal material, for example low-carbon steel. The metal shell 1 has a threaded portion 11 on the outer periphery thereof for fitting the spark plug S1 into the combustion chamber of the engine as described above.
The threaded potion 11 of the metal shell 1 has an outer diameter in a range of 12 to 14 mm This range corresponds to the range of M12 to M14 as specified in JIS (Japanese Industrial Standards).
The cylindrical insulator 2, which is made of alumina ceramic (Al2O3), is fixed and partially contained in the metal shell 1 such that an end 2a of the insulator 2 protrudes from an end 1a of the metal shell 1. The insulator 2 has a bore 2b that is formed through the insulator 2 in an axial direction of the insulator 2.
In this embodiment, the end 2a of the insulator 2 is away from the end 1a of the metal shell 1 by 2.5 mm in the axial direction of the insulator 2.
The center electrode 3 is made of a highly heat conductive metal material such as Cu as the core material and a highly heat-resistant, corrosion-resistant metal material such as a Ni (Nickel)-based alloy as the clad material.
The center electrode 3 is secured in the bore 2b of the insulator 2, so that it is electrically isolated from the metal shell 1. The diameter of the center electrode 3 decreases slightly in the bore 2b of the insulator 2 along the axial direction of the insulator 2 toward the end 2a of the same. For example, the maximum diameter of the center electrode 3 is equal to 2.3 mm, while the minimum one is equal to 2.25 mm. Further, the clearance between the outer surface of the center electrode 3 and the inner surface of the insulator 2 is made more than 50 μm at the end 2a of the insulator 2. Additionally, the diameter of the center electrode 3 may also be made constant in the bore 2b of the insulator 2 along the axial direction of the same.
Referring now to
The first diameter portion 3a of the center electrode 3 has a free end 3a1 and is positioned outside the bore 2b of the insulator 2.
In this embodiment, the end 3a1 of the first diameter portion 3a of the center electrode 3 is away from the end 2a of the insulator 2 by 1.5 mm in the axial direction of the insulator 2.
It is preferable that the first diameter portion 3a of the center electrode 3 is made up of a noble metal chip.
Using a noble metal chip, the space available for ignition in a first spark gap G1 is increased, while the noble metal chip is made not too thin to be easily worn down.
Further, the noble metal chip is preferably made of either a Pt-based alloy or an Ir-based alloy.
Specifying the material of the noble metal chip as above, a long service life is secured for the center electrode 3.
Furthermore, it is preferable that the noble metal chip has a diameter in a range of 0.3 to 1.6 mm and a length in the axial direction thereof in a range of 0.3 to 2 mm.
The second diameter portion 3c of the center electrode 3 has a diameter greater than that of the first diameter portion 3a.
The intermediate portion 3b of the center electrode 3 has, as shown in
The intermediate portion 3b of the center electrode 3 tapers from the second interface 3bc to the first interface 3ac with a taper degree, for example, of 110°.
Additionally, in
The first ground electrode 4 is made, for example, of a Ni-based alloy, it is column-shaped, for example an approximately L-shaped prism in this embodiment. Specially, the first ground electrode 4 has a cross section of approximately 2.8 mm×1.2 mm perpendicular to a length wise direction thereof.
The first ground electrode 4 has a base end that is joined, for example by welding, to the end 1a of the metal shell 1. The first ground electrode 4 also has a tip portion 4t that faces the end 3a1 of the first diameter portion 3a of the center electrode 3 through the first spark gap G1 in the axial direction of the insulator 2. In addition, the first spark gap G1 may have a size E of 1.1 mm.
It is preferable that the tip portion 4t of the first ground electrode 4 includes a noble metal chip 4a provided thereon as shown in
Using a noble metal chip, the space available for ignition in the first spark gap G1 is increased, while the noble metal chip is made not too thin to be easily worn down.
Further, the noble metal chip 4a is preferably made of either a Pt-based alloy or an Ir-based alloy.
Specifying the material of the noble metal chip as above, a long service life is secured for the first ground electrode 4.
On the other hand, each of the second ground electrodes 5 is made, for example, of a Ni-based alloy; it is column-shaped, for example an approximately L-shaped prism in this embodiment. Specially, each of the second ground electrodes 5 has a cross section of approximately 2.2 mm×1.2 mm perpendicular to a length wise direction thereof.
Each of the second ground electrodes 5 has a base end that is joined, for example by welding, to the end 1a of the metal shell 1. Each of the second ground electrodes 5 also has a tip portion 5t that has an end 5a.
As shown in
In this embodiment, as shown in
In the above-described spark plug S1, when the end 2a of the insulator 2 is not fouled with carbon deposits, normal sparks are discharged across the first spark gap G1, thereby igniting the air-fuel mixture.
However, when the end 2a of the insulator 2 of the spark plug S1 is fouled with carbon deposits due to incomplete burning of the air-fuel mixture, side sparks are discharged, instead of normal sparks, across the second spark gap G2.
Specifically, as shown in
After the insulation properties of the insulator 2 are recovered, normal sparks are again discharged across the first spark gap G1.
Having described the essential components and basic operation of the spark plug S1, the dimensional parameters designated as A, B, C, D, E, F, and H in
A is a minimum distance between the inner surface of the tip portion 2t of the insulator 2 and the end 5a of the tip portion St of the second ground electrode 5 in the radial direction of the insulator 2 (referred to as distance A hereinafter).
B is a minimum distance between the outer surface of the tip portion 2t of the insulator 2 and the end 5a of the tip portion 5t of the second ground electrode 5 in the radial direction of the insulator 22 (referred to as distance B hereinafter).
C is a distance between the end 2a of the insulator 2 and the second interface 3bc of the intermediate portion 3b of the center electrode 3 with the second diameter portion 3c of the same in the axial direction of the insulator 2 (referred to as distance C hereinafter).
D is a width of a cross section of the tip portion 5t of the second ground electrode 5 in a direction perpendicular to the axial direction of the insulator 2, the cross section being perpendicular to the radial direction of the insulator 2 (referred to as width D of the tip portion 5t of the second ground electrode 5 hereinafter).
E is a size of the first spark gap G1 between the end 3a1 of the first diameter portion 3a of the center electrode 3 and the tip portion 4t of the first ground electrode 4 in the axial direction of the insulator 2 (referred to as first spark gap size E hereinafter).
F is a length of the tip portion 2t of the insulator 2, which faces the end 5a of the tip portion 5t of the second ground electrode 5 in the radial direction of the insulator 2, in the axial direction of the insulator 2 (referred to as length F of the tip portion 2t of the insulator 2 hereinafter).
H is a width of a cross section of the tip portion 5t of the second ground electrode 5 in the axial direction of the insulator 2, the cross section being perpendicular to the radial direction of the insulator 2 (referred to as width H of the tip portion 5t of the second ground electrode 5 hereinafter).
The effective ranges of the above-defined parameters, which characterize the structure of the spark plug S1 according to the present embodiment, have been determined through experimental investigation and experience as follows.
First, the effective range of the distance C has been determined through experimental investigation.
It should be noted that a positive C (i.e., C>0) indicates that the second interface 3bc of the intermediate portion 3b of the center electrode 3 with the second diameter portion 3c of the same is positioned outside the bore 2 of the insulator 2, while a negative C indicates that the same is positioned inside the bore 2.
Sample spark plugs of seven different types were fabricated for the investigation. Those sample plug types had different distances C, but the same distances A and B. Specifically, for all the sample plug types, A=1.9 mm, B=0.5 mm, and accordingly, (A−B)=1.4 mm.
In the investigation, sample spark plugs were tested within a hermetically sealed chamber, where the pressure was kept at 0.6 Mpa. For each of the tested sample spark plugs, sparks were discharged at a frequency of 5 Hz, and the discharge path of the sparks was observed.
Accordingly, defining C<(A−B) for the spark plug S1, all side sparks will be discharged creeping through the end 2a of the insulator 2 of the spark plug S1, thereby recovering the insulation properties of the insulator 2.
Secondly, the effective range of a difference (A−B) between the distances A and B has been determined through experimental investigation.
It should be noted that the difference (A−B) corresponds to the radial thickness of the tip portion 2t of the insulator 2.
Sample spark plugs having different distances C and/or different differences (A−B) were fabricated. Specifically, for the sample spark plugs, the differences (A−B) of 0.6, 0.8, 1.0, 1.2, 1.4, and 1.6 mm were obtained by varying the distance A while fixing the distance B to 0.5 mm. The distances C of −0.2, 0, 0.2, 0.4, and 0.6 mm were used for the sample spark plugs.
Further, in the sample spark plugs, the first ground electrode 4 had been removed, so that only side sparks could be discharged between the center electrode 3 and the second ground electrode 5.
In the investigation, each of the sample spark plugs was installed to a direct-injection engine of 3000 CC and continuously operated at high load for 100 hours. After that, the condition of each of the sample spark plugs was checked as to whether channels were formed on the end 2a of the insulator 2 of the sample spark plug.
It can be seen from
This is because when the distance C in a spark plug is greater than zero, the discharge of side sparks is stared outside the bore 2 of the insulator 2 of the spark plug, thus alleviating the damage caused by side sparks to the end 2a of the insulator 2.
Further, when the difference (A−B) in a spark plug, which corresponds to the radial thickness of the tip portion 2t of the insulator 2 of the spark plug, is greater than 1.2 mm, the heat energy transferred from side sparks to the end 2a of the insulator 2 can be effectively dissipated, thus preventing formation of channels on the end 2a of the insulator 2.
Accordingly, defining 0<C and 1.2 mm<(A−B) for the spark plug S1, all side sparks will be discharged without formation of channels on the end 2a of the insulator 2 of the spark plug S1.
Thirdly, with respect to the width D of the tip portion 5t of the second ground electrode 5, it has been found that when (A−B)>2D, “inside sparks” tend to occur, instead of side sparks, when the insulator 2 of a spark plug is fouled with carbon deposits.
The inside sparks here denote sparks which creep along the outer surface of the insulator 2 toward the inside of an air pocket formed between the outer surface of the insulator 2 and the inner surface of the metal shell 1 and fly across the air pocket to an inside portion of the inner surface of the metal shell 1. Further, since the space for ignition in the inside of the air pocket is so small that ignition therein cannot be successful, it is required to prevent inside sparks from occurring.
Accordingly, it is preferable to define (A−B)<2D for the spark plug S1, so that the ignition capability of the spark plug S1 can be ensured.
Fourthly, with respect to the first spark gap size E, it is preferable to define E<A for the spark plug S1, so that normal sparks can be reliably discharged across the first spark gap G1 when the insulator 2 of the spark plug S1 is not fouled with carbon deposits.
Further, when E<B, inside sparks tend to occur, instead of side sparks, when the insulator 2 of the spark plug S1 is fouled with carbon deposits.
Accordingly, it is preferable to define B<E for the spark plug S1, so that the ignition capability of the spark plug S1 can be secured.
Finally, with respect to the length F of the tip portion 2t of the insulator 2 and the width H of the tip portion 5t of the second ground electrode 5, it is preferable to define F≦0.5H for the spark plug S1, so that the area of the end 5a of the tip portion 5t of the second ground electrode 5 which faces the tip portion 2t of the insulator 2 can be prevented from becoming too large, thereby suppressing formation of a fuel-bridge therebetween.
The fuel bridge here denotes a phenomenon in which the space between the end 5a of the tip portion 5t of the second ground electrode 2 and the outer surface of the tip portion 2t of the insulator 2 is filled with liquid fuel, thus forming a bridge of fuel across the space.
Further, it is necessary to define 0≦F for the spark plug S1, so that side sparks can be discharged which creep through the end 2a of the insulator 2, thereby recovering the insulation properties of the insulator 2.
Accordingly, it is preferable to define 0≦F≦0.5H for the spark pug S1, so that the insulation properties of the insulator 2 can be recovered when it is fouled with carbon deposits, while suppressing formation of the fuel bridge in the spark plug S1.
To sum up, the spark plug S1 according to the present embodiment includes a tubular metal shell 1, an insulator 2, a center electrode 3, a first ground electrode 4, and two second ground electrodes 5.
The metal shell 1 has a threaded portion 11 on an outer periphery thereof, which has an outer diameter in a range of 12 to 14 mm.
The insulator 2 is fixed in the metal shell 1. The insulator 2 has a bore 2b, which is formed through the insulator 2 in an axial direction of the insulator 2, and an end 2a that protrudes from an end 1a of the metal shell 1.
The center electrode 3 is secured in the bore 2b of the insulator 2. The center electrode 3 has a first diameter portion 3a, a second diameter portion 3c, and an intermediate portion 3b provided between the first and second diameter portions 3a and 3c. The first diameter portion 3a is positioned outside the bore 2b of the insulator 2 and has a free end 3a1. The second diameter portion 3c has a diameter greater than that of the first diameter portion 3a. The intermediate portion 3b has a first interface 3ab with the first diameter portion 3a and a second interface 3bc with the second diameter portion 3c and tapers from the second interface 3bc to the first interface 3ab. The second interface 3bc of the intermediate portion 3b with the second diameter portion 3c is positioned outside the bore 2b of the insulator 2.
The first ground electrode 4 has a base end joined to the end 1a of the metal shell 1 and a tip portion 4t that faces the end 3a1 of the first diameter portion 3a of the center electrode 3 through a first spark gap G 1 in the axial direction of the insulator 2.
Each of the second ground electrodes 5 has a base end joined to the end 1a of the metal shell 1 and a tip portion 5t that faces the second interface 3bc of the intermediate portion 3b of the center electrode 3 with the second diameter portion 3c of the same through a second spark gap G2 in a radial direction of the insulator 2. The tip portion 5t of each of the second ground electrodes 5 also faces in the radial direction of the insulator 2 a tip portion 2t of the insulator 2 which includes the end 2a of the insulator 2.
The spark plug S1 has an improved structure in which the dimensional parameters including the distance A, the distance B, and the distance C satisfy the following dimensional relationships:
0<C<(A−B); and
1.2 mm<(A−B).
With the above structure, when the insulator 2 of the spark plug S1 is fouled with carbon deposits, side sparks are discharged without formation of channels on the end 2a of the insulator 2, thereby recovering the insulation properties of the insulator 2.
In this embodiment, a spark plug S2 is provided which has a structure almost identical to that of the spark plug S1 according to the previous embodiment. Accordingly, only the difference in structure between the spark plugs S1 and S2 is to be described below.
As described previously, in the spark plug S1, the center electrode 3 has an intermediate portion 3b provided between the first diameter portion 3a and the second diameter portion 3c. The intermediate portion 3b has a first interface 3ab with the first diameter portion 3a and a second interface 3bc with the second diameter portion 3c and tapers from the second interface 3bc to the first interface 3ab. The second interface 3bc is positioned outside the bore 2b of the insulator 2 and faces the end 5a of the tip portion St of the second ground electrode 5 in a radial direction of the insulator 2.
In comparison, in the spark plug S2, the center electrode 3 has a first diameter portion 3a and a second diameter portion 3c, but no intermediate portion 3b provided therebetween.
As shown in
In the spark plug S2, the dimensional parameters A, B, D, E, F, and H, which influence the insulation properties of the insulator 2 and occurrence of the channeling problem, have the same definitions as in the spark plug S1.
However, in the spark plugs S1 and S2, the distance C has different definitions. In the spark plug S2, the distance C is defined as a distance between the end 2a of the insulator 2 and the interface 3ac between the first and second diameter portions 3a and 3c of the center electrode 3.
The inventor of the present invention has confirmed through experimental investigation that the dimensional relation ships defined for the spark plug S1 between the parameters A, B, C, D, E, F, and H are still useful, and accordingly can provide the same effects to the spark plug S2.
Thus, description of the dimensional relationships and the effects thereof is not repeated here.
While the above particular embodiments of the invention have been shown and described, it will be understood by those who practice the invention and those skilled in the art that various modifications, changes, and improvements may be made to the invention without departing from the spirit of the disclosed concept.
For example, in the previous embodiments, the center electrode 3 may have an intermediate portion 3b as shown in
Such modifications, changes, and improvements within the skill of the art are intended to be covered by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-161827 | May 2004 | JP | national |
