Description:Complete Specification

The following specification describes and ascertains the nature of this invention and the manner in which it is to be performed.

Field of the invention
[0001] This invention is related to a spark plug with an insulating layer.

Background of the invention
[0002] The normal operating voltage of spark plug ranges from 12kV to 30kV depending on an application. Housing is at ground potential and core of the insulator i.e. metal part will be at high positive/negative potential. Sometime due to increase in combustion pressure or due to increase in electrode gap, there will be a tendency to spark between the terminal stud and the housing. This phenomenon is called as head flashover. Usually, insulators have modified outer geometry in the head to increase the head flashover resistance.

[0003] A US patent application 20130229102 discloses a spark plug that includes a terminal for receiving an electrical charge, a firing electrode on a firing end of the spark plug, and a conductor electrically coupling the terminal and firing electrode. The spark plug further includes a casing, an electrical insulator, and a ground electrode. The casing is at least partially formed from an electrically conductive material and is configured to be electrically coupled to a ground. The electrical insulator separates the conductor from the casing. The ground electrode is positioned proximate to the firing electrode, but separated from the firing electrode by a spark gap. The ground electrode includes an extension projecting from the casing such that the firing electrode is closer to the ground electrode than the firing electrode is to the casing. There is substantially no air gap between the interior of the casing and the electrical insulator from the firing end of the spark plug.

Brief description of the accompanying drawing
[0004] Figure 1 illustrates a spark plug with an insulating layer in accordance with an embodiment of the invention.

Detailed description of the embodiments
[0005] Figure 1 illustrates a spark plug 10 with an insulating layer 22, in accordance with an embodiment of the invention. The spark plug 10 comprises a main electrode 12 having a terminal 13 for receiving electric power and a ground electrode 14 positioned in proximity to a tip 15 of the main electrode 12. The spark plug 10 comprises a spark gap 16 formed between the main electrode 12 and the ground electrode 14 and a casing 18 enclosing the main electrode 12. The spark plug 10 comprises an insulating layer 22 defined around an outer circumference of the main electrode 12 such that a triple point of the spark plug 10 is displaced by a predefined distance.

[0006] Further, the construction of the spark plug 10 and components of the spark plug 10 is explained in detail. The main electrode 12 comprising two terminals, a first terminal 13 for receiving the electric power and an opposite second terminal 15 being in proximity to the ground electrode 14, ignites the spark upon receiving the electric power. The casing 18 is made of an insulating material such as ceramic and the casing 18 covers the length of the main electrode 12. The partial discharge of voltage occurs along the surface of the casing 18. The first point of initiation of electron avalanche is usually observed at the triple point 20, i.e., where main electrode 12, casing 18 and atmospheric air comes in contact with each other. In the conventional spark plugs, the triple point 20 is present at the interface where the casing 18 and the main electrode 12 comes into contact. During an operational state of the spark plug 10, the spark will jump from the triple point 20 and travel through the casing 18 before reaching the ground electrode 14. In order to avoid this, the present invention provides a solution by shifting the triple point 20 to a higher displacement in the direction of the terminal 13 of the main electrode 12.

[0007] The insulating layer 22 shifts the triple point 20 in an upward direction towards the terminal 13 of the main electrode 12 which in turn increases a displacement between the triple point 20 and the casing 18. According to one embodiment of the invention, the triple point 20 is shifted by a few millimeters in the upward direction towards the tip/terminal 13 of the main electrode 12 (which receives the electric power).

[0008] According to one embodiment of the invention, the insulating layer 22 is made of a material chosen from a group of materials comprising a ceramic layer, PTFE, and the like. The insulating layer 22 comprises a first portion 10(a) and a second portion 10(b). The first portion 10(a) is positioned and secured to the outer circumference of the main electrode 12. The second portion10(b) of the insulating layer 22 is positioned on an outer circumference of the main electrode 12 and adjoins with the casing 18. According to one embodiment of the invention, a shape of the second portion 10(b) of the insulating layer 22 is made in accordance with a shape of the main electrode 12.

[0009] No airgap is formed between the casing 18 and the main electrode 12 due to the insulating layer 22. The insulating layer 22 blocks an access to flow of voltage at a first connection point of the casing 18 and the main electrode 12 during an operating state of the spark plug 10. With the new design of the insulating layer 22, there is an increase in the flash over resistance and reduction in the internal defect cost. The new design provides a low cost effective solution in avoiding the spark to jump in between the main electrode 12 and the ground terminal via the casing 18, but will allow power to travel along the length of the main electrode 12 and jump onto the ground electrode 14 via the spark gap 16.

[0010] Embodiments explained in the description above are only illustrative and do not limit the scope of this invention. Many such embodiments and other modifications and changes in the embodiment explained in the description are envisaged. The scope of the invention is only limited by the scope of the claims
, Claims:We Claim:
1. A spark plug (10) comprising:
- a main electrode (12) having a terminal (13) for receiving electric power;
- a ground electrode (14) positioned in proximity to a tip (15) of said main electrode (12);
- a spark gap (16) defined between said main electrode (12) and said ground electrode (14);
- a casing (18) enclosing said main electrode (12) ;
characterized in that:
- an insulating layer (22) defined around an outer circumference of said main electrode such that a triple point (20) of said spark plug (10) is displaced by a predefined distance.

2. The spark plug (10) as claimed in claim 1, wherein said insulating layer (22) displaces the triple point (20) towards the terminal (13), which in turn increases a displacement between the triple point (20) and said casing (18).

3. The spark plug (10) as claimed in claim 1, wherein said insulating layer (22) comprises a first portion (10(a)) and a second portion (10(b)), the first portion (10(a)) is defined between an inner circumference of said casing (18) and an outer circumference of said main electrode (12), and adjoins said main electrode (12).

4. The spark plug (10) as claimed in claim 3, wherein said second portion (10(b)) of said insulating layer (22) is defined on an outer circumference of said main electrode (12) and adjoins with said casing (18).

5. The spark plug (10) as claimed in claim 3, wherein a shape of said second portion 10(b) of said insulating layer (22) is in accordance with a shape of said casing (18) and a shape of said main electrode (12).

6. The spark plug (10) as claimed in claim 1, wherein the insulating layer (22) forms an air tight seal between said casing (18) and said main electrode (12).

7. The spark plug (10) as claimed in claim 1, wherein the insulating layer (22) prevents flow of electric current from said main electrode (12) to said casing (18) of said spark plug (10).