Spark plug

Details Spark plug Spark plug

2002-08-13

2004-03-16

Patel, Vip (Department: 2879)

Electric lamp and discharge devices

Spark plugs

Shaped electrode chamber, insulator end, shell skirt, baffle...

C313S141000

Reexamination Certificate

active

06707237

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a spark plug available as an ignition device for internal combustion engine, and more specifically to a spark plug capable of promoting rupture of fuel bridge if it should occur so as to fill a spark discharge gap, to thereby successfully suppress degradation in the ignition property.
DESCRIPTION OF THE BACKGROUND ART
Conventional spark plug generally comprises a center electrode protruded downward from the end face of an insulator, and a ground electrode joined at one end thereof to a metal shell, and is composed so as to form a spark discharge gap between the end face of the center electrode and ground electrode, where electric spark generated in the gap ignites mixed fuel gas. To improve startability of the spark plug at low temperature, it has been a general practice for internal combustion engine to raise concentration of fuel-air mixture sucked into a combustion chamber.
Suction of a fuel-air mixture of higher concentration aiming at improving the startability at lower temperature, however, tends to cause accumulation of the fuel in a liquid state within pistons. The accumulated fuel may adhere on the surface of the spark plug or fill the spark discharge gap in conjunction with reciprocating motion of the pistons at the starting, which results in formation of fuel bridge at the spark discharge gap. Since the fuel is electro-conductive, such formation of fuel bridge at the spark discharge gap will be causative of leakage of current even if a high voltage is applied to the spark discharge gap, and will prevent electric spark from being generated at the spark discharge gap. The fuel-air mixture after sucked into a combustion chamber will therefore not be ignited, which undesirably degrade the startability contrary to expectation.
It is therefore an object of the present invention to provide a spark plug which is less causative of the fuel bridge at the spark discharge gap even when the above-described, hyper-concentration, fuel-air mixture is supplied.
SUMMARY OF THE INVENTION
The present invention relates to a spark plug which comprises an insulator (
1
) having a center through hole (
1
D); a center electrode (
2
) disposed in the center through hole (
1
D) and extends along the direction of axial line (O); a metal shell (
5
) having a screw (
5
B) for assembling an internal combustion engine provided external of the insulator (
1
); and a ground electrode (
11
) joined at one end thereof through a joint portion (
55
) to the metal shell (
5
), and on the other end of which having a discharge plane (
111
A) being arranged so as to oppose to an end face (
22
B) of the center electrode (
2
) to thereby form a spark discharge gap (g); and which spark plug is characterized,
wherein the insulator (
1
) is engaged to the metal shell (
5
) through an engagement portion (
15
), and the center electrode (
2
) is protruded out from an end face (
1
E) of the insulator (
1
) and has formed therein a divergent portion (G), having a diameter increasing towards the end, on the end side beyond such engagement portion (
15
) and between the outer peripheral surface of the center electrode (
2
) and the inner peripheral surface of the insulator (
1
); Moreover, the end of the center electrode (
2
) forming the spark discharge gap (g) comprises a noble metal member (
22
) having a straight rod portion (
22
A) of 1.0 mm or less in diameter and 0.2 mm or more in length; and
assuming a primary intersectional line (PKL) as being defined as an intersectional line formed between the end face (
22
B) of the center electrode (
2
) or a plane (P
1
) extended therefrom and a lateral plane (
22
S) of the straight rod portion (
22
A) or a cylindrical plane (C
1
) extended therefrom;
further assuming a secondary intersectional line (SKL) as being defined as an intersectional line formed between the discharge plane (
111
A) or a plane (P
2
) extended therefrom and an end face (
112
B) of the ground electrode (
11
) or a plane extended therefrom;
further assuming a primary virtual line (PVL) as being defined as a virtual line containing a primary intersectional point (PP) and being in parallel to a virtual center axial line (O) of the spark plug referring to the screw (
5
B) for assembling the internal combustion engine,
wherein the primary intersectional point (PP) is a first point encountered the primary intersectional line (PKL)when a standard line (SL) parallel to the virtual center axial line (O) is moved across the spark discharge gap (g) to the joint portion
55
of the ground electrode (
11
) from the side opposite to such joint portion (
55
) placing the virtual center axial line (O) in between; and
further assuming a secondary virtual line (SVL) as being defined as a virtual line containing a secondary intersectional point (SP) and being in parallel to the virtual center axial line (O),
wherein the secondary intersectional point (SP) is a last point where the standard line (SL) similarly moved intersects with the primary intersectional line (PKL);
a width of coverage (K) as being defined as a distance between the primary virtual line (PVL) and secondary intersectional line (SKL) is set so as to satisfy a relation of
−
d≦K≦
0.5 (in mm)  (1)
(in mm: where d represents the diameter of the end face (
22
B) of the center electrode (
2
); and sign for K is defined as negative when the secondary intersectional line (SKL) stands closer to the joint portion (
55
) than the primary virtual line (PVL), and as positive when stands further); and
the width (w) of a portion of the discharge plane (
111
A) which falls within a range (WDS) between the secondary virtual line (SVL) and primary virtual line (PVL) satisfies a relation of
w<
2.1
−K
(in mm)  (2)
where K is the foregoing width of coverage.
It should now be noted that reference numerals and alphabets assigned to the individual constituents given in the Claims of the invention and in this section (SUMMARY OF THE INVENTION) were quoted from those used for the corresponded constituents shown in the attached drawings (
FIGS. 1
,
2
and
12
), which are merely for the purpose of facilitating understanding of the present invention, and by no means limit the concept of the individual constituents in the present invention.
According to the foregoing constitution, the fuel bridge is likely to rupture even if it should occur at the spark discharge gap, since areas for the fuel contact on the ground electrode and center electrode are reduced. More specifically, at the beginning of the operation, a starter motor cranks to allow admission of fuel-air mixture into a combustion chamber. Although use of a hyper-concentrated fuel-air mixture inevitably causes the fuel bridge at the spark discharge gap in conjunction with the motion of the pistons at the start time, the fuel bridge in the spark plug of the present invention is likely to rupture by vibration applied when the cranking is further sustained.
The spark plug is generally attached to an internal combustion engine so as to direct the side of the spark discharge gap downward. The fuel bridge generated at the spark discharge gap is sustained so that the liquid droplet of the fuel is suspended by adhesive force effected between such liquid droplet and the center electrode. Since the spark plug is designed so as to reduce the diameter of the end of the center electrode as small as 1.0 mm or less, which reduces an area for retaining the fuel droplet, so that the bridge will readily be ruptured even if it should undesirably be formed. It is also worth while pointing out that the center electrode has a straight rod portion of 0.2 mm or above in length, and that the rear side of such portion is connected to a divergent portion of the center electrode. Since the fuel bridge once formed with a hyper-concentrated fuel-air mixture extends over the side face of the center electrode, so that the elongation of the straight rod portion is advantageous in that preventing the fuel from spreading over a transitional portion towards

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