Conventionally, a combustion apparatus, such as an internal combustion engine, uses a spark plug for igniting an air-fuel mixture through spark discharge. In recent years, in order to meet demand for high output and low fuel consumption, a plasma jet ignition plug has been proposed, since the plasma jet ignition plug provides quick propagation of combustion and can more reliably ignite even a lean air-fuel mixture having a higher ignition-limit air-fuel ratio.
Generally, the plasma jet ignition plug includes a tubular insulator having an axial hole, a center electrode inserted into the axial hole in such a manner that a front end surface thereof is located internally of a front end surface of the insulator, a metallic shell disposed externally of the outer circumference of the insulator, and an annular ground electrode joined to a front end portion of the metallic shell. Also, the plasma jet ignition plug has a space (cavity) defined by the front end surface of the center electrode and a wall surface of the axial hole. The cavity communicates with an ambient atmosphere via a through hole formed in the ground electrode.
Additionally, such the plasma jet ignition plug ignites an air-fuel mixture as follows. First, voltage is applied between the center electrode and the ground electrode, thereby generating spark discharge therebetween and thus causing dielectric breakdown therebetween. In this condition, high-energy current is applied between the center electrode and the ground electrode for effecting transition of a discharge state, thereby generating plasma within the cavity. The generated plasma is blown off through an opening of the cavity, thereby igniting the air-fuel mixture.
Meanwhile, according to a conceivable method for achieving enhanced ignition performance, current having higher energy is applied after generation of spark discharge for generating a larger plasma jet. However, when such high-energy current is applied, the center electrode becomes likely to erode, potentially resulting in an abrupt increase in voltage required for generation of spark discharge.
According to a known method for coping with the above problem (for example, see Japanese Patent Application Laid-Open (kokai) No. 2007-287666, hereinafter “Patent Document 1”), the wall of the cavity has a stepped shape for imparting a throttle to the cavity, whereby even when current having relatively low energy is applied, excellent ignition performance can be achieved. Also, according to a proposed technique (for example, see Japanese Patent Application Laid-Open (kokai) No. 2006-294257, hereinafter “Patent Document 2”), the axial length of the cavity is relatively long for increasing the blown-off velocity of plasma, whereby the blown-off length of flame is increased, thereby improving ignition performance.
However, in association with the phenomenon (so-called channeling) that spark discharge erodes a portion of the insulator located on a spark discharge path, the technique described in Patent Document 1 involves the following problem: since the wall of the cavity is curved (bent), the insulator is apt to be eroded at the curved (bent) portion. Further, since a spark discharge path which passes through an eroded portion of the insulator becomes shorter than other spark discharge paths, spark discharge is generated in a concentrated manner along the spark discharge path, causing local concentration of channeling. As a result, the insulator is eroded in a deep streaky manner. Thus, a groove lying on a line which connects the center electrode and a portion of the ground electrode located toward the outer circumference may be formed on the wall of the cavity. Spark discharge is generated along this groove. Even though plasma is generated, the plasma is less likely to be blown off outward due to the existence of the ground electrode. That is, according to the technique described in Patent Document 1, excellent ignition performance can be achieved at an early stage, but ignition performance may drastically deteriorate in the course of use.
Meanwhile, when, as in the case of the technique described in Patent Document 2, the axial length of the cavity is relatively long, the distance between the center electrode and the ground electrode becomes relatively long. Thus, a discharge voltage required for generation of spark discharge increases, causing rapid erosion of the center electrode and the insulator. As a result, ignition performance deteriorates rapidly, and difficulty may be encountered in generating spark discharges over a long period of time.