Plasma jet ignition plug

#The present invention relates to a plasma jet ignition plug.

#Conventionally, a spark plug has been used to ignite an air-fuel mixture through spark discharge (may be referred to merely as “discharge”) for operation of an engine, such as an automotive internal combustion engine. In recent years, high output and low fuel consumption have been required of internal combustion engines. To fulfill such requirements, development of a plasma jet ignition plug has been conducted, since the plasma jet ignition plug provides quick propagation of combustion and exhibits such high ignition performance as to be capable of reliably igniting even a lean air-fuel mixture having a higher ignition-limit air-fuel ratio.

#The plasma jet ignition plug has a structure in which an insulator formed from ceramic or the like surrounds a spark discharge gap between a center electrode and a ground electrode, thereby forming a small-volume discharge space called a cavity. An example system of ignition of the plasma jet ignition plug is described. For ignition of an air-fuel mixture, first, high voltage is applied between the center electrode and the ground electrode, thereby generating spark discharge. By virtue of associated occurrence of dielectric breakdown, current can be applied between the center electrode and the ground electrode with a relatively low voltage. Thus, through transition of a discharge state from the spark discharge effected by further supply of energy, plasma is generated within the cavity. The generated plasma is jetted out through an opening (so-called orifice), thereby igniting the air-fuel mixture (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2006-294257, “Patent Document 1”).

#FIG. 1 is an explanatory, partially sectional view showing the configuration of a plasma jet ignition plug according to an embodiment of the present invention.

#FIG. 2 is an explanatory sectional view showing essential portions of the plasma jet ignition plug of FIG. 1.

#FIG. 3A is an explanatory sectional view of a plasma jet ignition plug as cut by a plane which cross-sections contact portions and is orthogonal to the axial direction.

#FIG. 1 shows a plasma jet ignition plug according to an embodiment of the present invention. FIG. 1 shows, partially in section, the configuration of a plasma jet ignition plug 1 according to the embodiment of the present invention. FIG. 2 shows, in section, essential portions of the plasma jet ignition plug 1. In the following description with reference to FIGS. 1 and 2, a downward direction on the paper on which FIG. 1 appears is referred to as a frontward direction along an axis O, and an upward direction on the paper is referred to as a rearward direction along the axis O.

#As shown in FIGS. 1 and 2, the plasma jet ignition plug 1 includes a substantially tubular insulator 4 having an axial bore 3 extending in the direction of the axis O, a center electrode 2 accommodated within the axial bore 3 of the insulator 4 on a front side, a metal terminal 20 whose portion is accommodated within the axial bore 3 of the insulator 4 on a rear side, a metallic shell 5 holding the insulator 4, and a ground electrode 6 joined to the metallic shell 5 and disposed frontward of the insulator 4.

#The insulator 4 is a substantially cylindrical insulation member formed from alumina or the like by firing and having the axial bore 3 along the direction of the axis O. The insulator 4 has a flange portion 7 having the largest outside diameter and formed substantially at its center with respect to the direction of the axis O. The insulator 4 has a rear trunk portion 8 extending rearward of the flange portion 7 and having an outside diameter smaller than that of the flange portion 7. The rear trunk portion 8 accommodates therein a portion of the metal terminal 20. The insulator 4 has a front trunk portion 9 extending frontward of the flange portion 7 and having an outside diameter smaller than that of the flange portion 7. The front trunk portion 9 accommodates therein a seal body 25 and a portion of the center electrode 2. Furthermore, the insulator 4 has an insulator front end portion 14 having an outside diameter smaller than that of the front trunk portion 9 and extending frontward of the front trunk portion 9 via an insulator taper portion 11 whose outside diameter reduces frontward from the front trunk portion 9. The insulator front end portion 14 accommodates therein the rest of the center electrode 2. The axial bore 3 of the insulator 4 extends frontward from the rear end of the insulator 4 as follows: the axial bore 3 extends through the rear trunk portion 8, the flange portion 7, and the front trunk portion 9 while having substantially the same inside diameter; the axial bore 3 extends through the front trunk portion 9 and reduces frontward in inside diameter so as to form an insulator ledge 15; and the axial bore 3 extends from the insulator ledge 15 through the insulator front end portion 14 while having a smaller inside diameter than in the front trunk portion 9. The insulator front end portion 14 has a second insulator ledge 17 located frontward of the insulator ledge 15 and formed with its inside diameter being further reduced frontward. The insulator front end portion 14 accommodates therein a forefront portion 23 of the center electrode 2 at a position located frontward of the second insulator ledge 17. The inner circumferential surface of the insulator front end portion 14, which accommodates therein the forefront portion 23, and a front end surface 40 of the forefront portion 23 define a discharge space called a cavity 50.