A spark plug is mounted to an internal combustion engine or the like and used for igniting an air-fuel mixture or the like in a combustion chamber. Generally, a spark plug includes an insulator having an axial hole, a center electrode inserted into a forward portion of the axial hole, a terminal electrode inserted into a rear end portion of the axial hole, a metallic shell provided on the outer circumference of the insulator, and a ground electrode fixed to a forward end portion of the metallic shell. Also, a gap is formed between a forward end portion of the center electrode and a distal end portion of the ground electrode, and voltage is applied to the center electrode (gap) for generating spark discharges across the gap, thereby igniting the air-fuel mixture or the like.
Also, in order to restrain radio noise generated in association with operation of an internal combustion engine or the like, a resistor can be provided in the axial hole between the center electrode and the terminal electrode (refer to, for example, Japanese Patent Application Laid-Open (kokai) No. 2006-66086, Japanese Patent Application Laid-Open (kokai) No. 2005-327743, etc.). Generally, the resistor is formed through compressional heating of a resistor composition which contains carbon as an electrically conductive material, glass powder, ceramic particles, etc. The formed resistor contains glass and carbon and is in a state of phase separation in which an interstitial phase composed primarily of molten glass exists around a particulate aggregate phase, and the interstitial phase contains carbon and ceramic particles. The center electrode and the terminal electrode are electrically connected through electrically conductive paths formed of carbon in the interstitial phase.
In recent years, in order to improve ignitability, there has been proposed a spark plug in which an interelectrode insert (including a resistor) disposed between the forward end of the terminal electrode and the rear end of the center electrode has a relatively low resistance. In such a spark plug, since relatively large current flows through the interelectrode insert (resistor) at the time of occurrence of spark discharges, the electrically conductive paths formed in the resistor are likely to have a high temperature. Furthermore, particularly, at the forward portion of the interelectrode insert which is disposed toward a combustion chamber and is particularly likely to have a high temperature in the course of use, coupled with flow of a relatively large current, the electrically conductive paths have a very high temperature, potentially resulting in rapid oxidation. As a result, in the course of use, the resistance of the interelectrode insert (resistor) may abruptly increase. That is, a spark plug having the interelectrode insert of a relatively low resistance encounters difficulty in securing a good under-load life characteristic.
Also, in association with recent tendency toward higher outputs of engines, etc., demand has been rising for further improvement of durability and restraint of radio noise.
The present disclosure has been conceived in view of the above circumstances, and a first advantage thereof is to reliably implement an excellent under-load life characteristic for a spark plug whose interelectrode insert has a relatively low resistance and which thus encounters difficulty in securing a good under-load life characteristic. A second advantage of the present disclosure is to improve restraint of radio noise and the life of a resistor.