Patent ID: 6858974
Filing Date: 2005-02-22
Classification: H01T

Abstract:
1. A spark plug installed to face into a combustion chamber of a cylinder of an internal combustion engine and to produce a spark discharge that ignites air-fuel mixture in the combustion chamber and causes combustion such that an ionic current flowing during combustion can be utilized for detecting misfire of the engine, comprising: a core connected to an ignition coil; a center electrode made of at least one of iridium and an iridium alloy and connected to the core through a seat; and a ground electrode grounded at one and separated at other end from the center electrode with a gap; wherein the improvement comprises: a sum of surface areas of the seat the center electrode is defined as not less than a prescribed value and remains unchanged when the surface area of the center electrode is decreased by increasing the surface area of the seat relative to the amount of the decrease, and when the surface area of the center electrode is increased by decreasing the surface area of the seat relative to the amount of the increase.