Abstract
In the field of automotive gasoline engines, new products aiming at greater fuel economy and cleaner exhaust gases are under development with the aim of preventing environmental destruction. Severe ignition environments such as lean combustion, stronger charge motion, and large quantities of EGR require ever greater combustion stability. In an effort to meet these requirements, an iridium plug has been developed that achieves high ignitability and long service life through reduction of its diameter, using a highly wear-resistant iridium-rhodium alloy as the center electrode.
Recently, direct injection engines have attracted attention. Though the direct injection engines of each manufacturer have unique features, most of them alternate between two modes of combustion: stratified and homogeneous combustions. In the stratified combustion, in particular, although various air flows such as tumble and swirl exist, all of them are designed to bring excessively rich air-fuel mixtures to the plug ignition region. Therefore, carbon is likely to be generated during combustion. This necessitates plug carbon fouling resistance. Carbon fouling is a phenomenon in which carbon deposited on the plug surface causes insulation deterioration, thereby allowing discharge to run along the insulator surface deep into the housing pocket. The sparks from this phenomena have very poor ignitability, and may even cause engine misfire.
So, we set the goal for developing a spark plug that has non carbon fouling structure, as well as high ignitability, and longer life. And we have reached a conclusion to develop a new concept of the 3-ground electrodes plug which makes a significant improvement in carbon fouling resistance while keeping the advantages of the iridium plug, achieving maintenance-free condition. The structure of this plug is unique in having a pair of side ground electrodes at the corner of the insulator top, in addition to the ground electrode of the conventional single type. Thus, this plug has two side gaps for carbon cleaning in addition to the ordinary spark gap. On this plug, discharge normally takes place between the center electrode and the main ground electrode. However, if the insulation on the end surface of the insulator deteriorates, discharge will take place between the center electrode and the side ground electrodes through the end surface of the insulator, burning carbon to restore insulation. This brings discharge back to the main gap. This mechanism always maintains insulation on the insulator surface. The key point of the development is that the side gap spark occurs only under carbon fouling condition by optimum position of the side ground electrode.
This paper outlines the current iridium plug, and describes the characteristics of the newly developed iridium plug with improved carbon fouling resistance.