The present invention relates to a plasma-jet spark plug that produces a plasma by a spark discharge to ignite an air-fuel mixture in an internal combustion engine. The present invention also relates to an ignition system using the plasma-jet spark plug.
A spark plug is widely used in an automotive internal combustion engine to ignite an air-fuel mixture by a spark discharge. In response to the recent demand for high engine output and fuel efficiency, it is desired that the spark plug increase in ignitability to show a higher ignition-limit air-fuel ratio and achieve proper lean mixture ignition and quick combustion.
Japanese Laid-Open Patent Publication No. 2-72577 discloses, as one example of high-ignitability spark plug, a plasma-jet spark plug that has a pair of center and ground electrodes defining therebetween a discharge gap and an electrical insulator surrounding the discharge gap so as to form a discharge cavity within the discharge gap. In the plasma-jet spark plug, a spark discharge is generated through the application of a high voltage between the center and ground electrodes. A phase transition of the discharge occurs by a further energy supply to eject a plasma from the discharge cavity for ignition of an air-fuel mixture in an engine combustion chamber.
The plasma can be ejected in various geometrical forms such as flame form. The plasma in flame form (occasionally referred to as “plasma flame”) advantageously extends in an ejection direction and secures a large contact area with the air-fuel mixture for high ignitability.