CLIAMS:We Claim:
1. A spark plug for a spark ignited engine, said spark plug comprising an insulated electrode, characterized in that:
an integrated first member for enclosing a part of said insulated electrode, comprising a first engaging feature; and
a removable second member in connection with said first member for fastening said spark plug to said spark ignited engine, comprising a second engaging feature.

2. The spark plug as claimed in claim 1, wherein said first engaging feature is selected from a group comprising a groove, a plurality of teeth and an external thread.

3. The spark plug as claimed in claim 1, wherein said second engaging feature is selected from a group comprising a ridge, a plurality of teeth and an internal thread.

4. The spark plug as claimed in claim 1, wherein said first member engages with said second member by mating of said first engaging feature and said second engaging feature.

5. The spark plug as claimed in claim 1, wherein shear strength of said second engaging feature is less than shear strength of said first engaging feature.

6. The spark plug as claimed in claim 1, wherein said first member and said second member enables transmission of optimal torque to said spark plug during fastening to said spark ignited engine.

7. The spark plug as claimed in claim 1, wherein said second engaging feature shear off on engagement with said first engaging feature on application of torque beyond a threshold torque.

8. A spark plug for a spark ignited engine, said spark plug comprising an insulated electrode, wherein a part of said insulated electrode is enclosed in a threaded metallic shell for mounting to said spark ignited engine, characterized in that:
an polygonal nut integral to said insulated electrode, wherein edges of said polygonal nut are rounded.

9. The spark plug as claimed in claim 8, wherein said polygonal nut is a hexagonal nut.

10. The spark plug as claimed in claim 8, wherein said rounded edges avoids transmission of excess torque to said spark plug.
,TagSPECI:The following specification particularly describes the invention and the manner in which it is to be performed.

Field of the invention:
[001] The invention relates to prevention of over-tightening of a spark plug in a spark ignited engine.
Background of the invention:
[002] A spark plug is used for initiation of combustion (Ignition) by passing a high voltage spark, inside the combustion chamber filled with combustible mixture of air and fuel. The normal fuels used are Petrol, Compressed Natural Gas (CNG), Liquefied Petroleum Gas (LPG) and Kerosene. A general spark plug includes a center electrode, an insulating member which holds the center electrode in an axial hole, a metal shell which surrounds the periphery of the insulating member to hold the insulating member, and a ground electrode which is joined to the metal shell at a proximal end portion and which forms a spark gap at a distal end portion with the center electrode. The fuel mixture is ignited by a spark discharge occurring in the spark gap.
[003] According to existing methods to prevent over tightening of spark plug, a torque wrench with pre-set torque values is used. When a pre-set tightening torque is reached in such torque wrenches, the torque wrench gives an indication (alarm) or does not tight over that torque value.
Brief description of the accompanying drawings:
[004] An embodiment of the invention is described with reference to the following accompanying drawings,
[005] Fig. 1 illustrates a perspective view of an engagement between a first member and a second member of a spark plug through mating of a groove and a ridge, in accordance to an embodiment of the present invention;
[006] Fig. 2 illustrates a front cross sectional view of an engagement between the first member and second member of the spark plug through threads, in accordance to an embodiment of the present invention; and
[007] Fig. 3 illustrates a front view and a top view of a spark plug with an integrated polygonal nut, in accordance to an embodiment of the present invention.
Detailed description of the embodiments:
[008] Fig. 1 illustrates a perspective view of an engagement between a first member and a second member of a spark plug through mating of a groove and a ridge, in accordance to an embodiment of the present invention. A spark plug 100 for a spark ignited engine is provided. The spark plug 100 comprises a first member 104 and a second member 106. The first member 104 is integrated to and encloses/holds a part of an insulated central electrode. The insulation to the central electrode (not shown in Fig. 1) is provided by an insulating material 102 such as ceramic and the like. The first member 104 also comprises a threaded portion 110 and a first engaging feature 114. A circular disk/collar 112 is connected between the threaded portion 110 and the first engaging feature 114. A gasket (not shown in Fig. 1) below the circular disk 112 defines the boundary until which the spark plug 100 is inserted inside the engine cylinder. The threaded portion 110 of the spark plug 100 is metallic in nature and is used for fastening/mounting to the spark ignited engine cylinder. Also, a ground electrode 108 is connected to the threaded portion 110. The first engaging feature 114 is in connection with the second member 106 through a second engaging feature 116. The second member 106 is a polygonal housing or a nut. Also, the second member 106 is a removable component of the spark plug 100. The engagement between the first member 104 and the second member 106 occurs above the circular disk 112 i.e. out of the cylinder of the spark ignited engine. The second member 106 is used for fastening the spark plug 100 to the spark ignited engine. The first engaging feature 114 is selected from a group comprising but not limited to a groove, a plurality of teeth and an external thread. Similarly, the second engaging feature 116 is selected from a group comprising but not limited to a protrusion, a ridge, a plurality of teeth and an internal thread.
[009] According to an embodiment of the present invention and as shown in the enlarged and encircled portion of Fig. 1, the first member 104 engages with the second member 106 by mating of the first engaging feature 114 and the second engaging feature 116. The first engaging feature 114 is provided on an outer periphery of the first member 104. Similarly, the second engaging feature 116 is provided on inner periphery of the second member 106. When assembled, the second engaging feature 116 slides into the first engaging feature 114 and fits with no relative motion between the first member 104 and the second member 106.
[0010] According to an embodiment of the present invention, shear strength of the second engaging feature 116 is less than the shear strength of the first engaging feature 114 and the threaded portion 110. In another embodiment, the shear strength of the first engaging feature 114 is less than the shear strength of the threaded portion 110 in the case where the first member 104 and the threaded portion 110 are made from different material. The engagement of the first member 104 with the second member 106 enables transmission of optimal torque to the spark plug 100 during fastening or mounting to the spark ignited engine. Thus, the tightening torque to the spark plug 100 is provided indirectly by tightening the second member 106 in engagement with the first member 104. So, when the spark plug 100 is to be tightened, then a spanner or wrench is fixed mating with the second member 106. When a torque is applied, the torque is transmitted from the second member 106 to the first member 104 through the groove and ridge or plurality of intermeshing teeth. The groove, ridge and the plurality of intermeshing teeth are provided in axial direction of the spark plug 100.
[0011] According to an embodiment of the present invention, the second engaging feature 106 shear off or breaks on application of a torque beyond or more than a threshold torque. The second engaging feature 116 is designed in a manner in which, torque at which shearing off occurs, is less than the thread yielding torque of the threaded portion 110 of the spark plug 100.
[0012] According to an embodiment of the present invention, plurality of teeth is used instead of a ridge on the first member 104. Similarly, a mating plurality of teeth is provided on the second member 106. The intermeshing teeth enable a torque to be transmitted to the spark plug 100.
[0013] Fig. 2 illustrates a front cross sectional view of an engagement between the first member and second member of the spark plug through threads, in accordance to an embodiment of the present invention. An enlarged and encircled portion of the Fig. 2 shows the engagement of first member 104 with the second member 106. The first engaging feature 114 of the first member 104 comprises an external thread and encloses a part of the insulated central electrode. The insulation is performed by using a suitable insulating material 102. The second engaging feature 116 of the second member 106 comprises an internal thread. The internal thread mates with the external thread and enable the contact of second member 106 and the first member 104. Further, the shear strength of the second engaging feature 116 i.e. the internal thread is less than the shear strength of the first engaging feature 114 i.e. the external thread and the threaded portion 110 of the spark plug 100. When a torque wrench is used for fastening the spark plug 100 to the spark ignited engine, the torque is applied to the second member 106. The torque is transmitted from the second member 106 to the integrally connected first member 104, and in turn transmits the torque to the threaded portion 110. The threaded portion 110 is provided with a ground electrode 108. A spark is generated in the gap formed between the tip of the central electrode 202 and the ground electrode 108.
[0014] According to an embodiment of the present invention, since the shear strength of the second member 106 is the less than the shear strength of the first member 104, the second engaging feature 116 shears off, if a torque more than (or in excess) the designated threshold torque is applied.
[0015] According to an embodiment of the present invention, the shearing off of the second engaging feature 116 conform the application of excess torque. In accordance to another embodiment of the present invention, the shearing off of the second engaging feature 116 conform the optimal tightening of the spark plug 100.
[0016] Fig. 3 illustrates a front view and a top view of a spark plug with an integrated polygonal nut, in accordance to an embodiment of the present invention. A spark plug 300 for a spark ignited engine comprises but not limited to a central electrode insulated by an insulating material 102. A part of the insulated central electrode is further enclosed by a first member 104. The first member 104 is a metallic shell with a threaded portion 110 for mounting to the spark ignited engine. The spark plug 300 further comprises an integral polygonal nut 302. The polygonal nut 302 is affixed to and a part of the first member 104. The edges of the polygonal nut 302 are rounded. The polygonal nut 302 is a hexagonal nut. Further, a polygonal nut 302 is selected from a group comprising a triangular nut, square nut, pentagon nut and the like. The rounded edges of the polygonal nut 302 avoid transmission of excess torque to spark plug 300 and thereby prevent thread yielding of the spark plug 300. When a torque more than a threshold torque is applied, then because of the round design of the polygonal nut 302, the torque wrench slips out. The slipping of the torque wrench or spanner ensures the protection of spark plug 300 against the over tightening. Also, the thread yielding of the spark plug 300 or the spark ignited engine is prevented.
[0017] A top view of the spark plug 300 is shown below the front view of the same spark plug 300 in the Fig. 3. The rounded edges 304 of the polygonal nut 302 are shown in the top view of the spark plug 300.
[0018] According to an embodiment of the present invention, when the spark plug is tightened more than the safe limit or threshold limit, the second engaging feature shears off instead of transmitting the shearing force to the first engaging feature. Thereby the first member is not subjected to high torque and thus thread yielding of the spark plug is prevented. Further, when the second member starts rotating freely after applying high torque, the free rotation corresponds to shearing off of the second engaging feature. The second member is removed and replaced with a second member without replacing the entire spark plug. Furthermore, as the second member is removable, for the same spark plug, by using different second member, the spark plug is made to be modified to fit different sized spanners or wrenches. Still further, the assembly of the first member and the second member prevents a liability claims by a customer or a user from mishandling of spark plugs during tightening to engines.
[0019] It should be understood that 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.