Spark plug resistant to accidental discharges

#This invention relates to a spark plug in which an insulator is supported by resting its stepped portion on a shoulder portion of a metallic shell.

#In a spark plug for an internal combustion engine, an insulator has generally been made of aluminum oxide (Al.sub.2 O.sub.3) which is inferior in heat-conductivity.

#With high speed and high power requirement of the internal combustion engine, a front end of the insulator tends to be exposed to higher ambient temperature so as to cause preignition. In order to prevent the spark plug against the preignition, it is suggested in an integral type of the spark plug that the insulator is integrally made of aluminum nitride (AlN) superior in heat-conductivity so as to prevent preignition within an extensive range of running condition of the internal combustion engine.

#FIG. 1 is a longitudinal half-sectional view of a spark plug according to an embodiment of the present invention, but its upper part is broken away;

#FIG. 2 is a full longitudinal half sectional view of a spark plug according to the same embodiment of the invention as FIG. 1;

#FIG. 3 is a longitudinal half-sectional view of a previous in-house spark plug and employed for the purpose of carrying out endurance test; and

#Referring to FIGS. 1 and 2 which substantially show a spark plug 100, the spark plug 100 has a cylindrical metallic shell 2, to a front end of which a L-shaped ground electrode 1 is fixedly attached by means of welding. Within the metallic shell 2, is a tubular insulator 3 placed, an inner space of which serves as an axial bore 31. Within the bore 31, a center electrode 4 is concentrically placed.

#An outer surface of the metallic shell 2 has a diameter-increased hexagonal head 21, a barrel portion 22 and a threaded portion 23 which is to be attached to a cylinder block of an internal combustion engine. Each inner diameter of the hexagonal head 21 and the barrel portion 22 is enlarged, while an inner diameter of the threaded portion 23 is smaller. An inner wall of the threaded portion 23 has a shoulder portion 24 on which a stepped portion 3a rests by way of a gasket 25. The stepped portion 3a is provided with the insulator 3 to support the insulator 3 within the metallic shell 2. The shoulder portion 24 includes an upper shoulder and lower shoulder as indicated at (a), (b) in FIG. 1.

#As illustrated in FIG. 2, the insulator 3 has a tapered leg portion 32, a diameter-increased portion 33 and a head 35 whose outer surface has a corrugation 34. Between the diameter-increased portion 33 and the leg portion 32, is a middle portion 36 provided which is diametrically somewhat larger than the leg portion 32. The insulator 3 has a rear half 6 made of aluminum oxide (Al.sub.2 O.sub.3) and a front half 5 made of aluminum nitride (AlN) so as to form a split-type insulator. The front half 5 includes the middle portion 36. With a rear end surface of the middle portion 36, is a cylindrical recess 51 provided which extends to the leg portion 32 to form a housing 51a in a manner to surround the recess 51.