The lean burn gas engine ignited by pilot oil is a promising alternative combustion system [1]. First, pilot flame is auto-ignited, then ignition source propagate toward lean gas mixture. Despite much research work conducted experimentally [2] [3], little phenomenological understanding has been made on ignition characteristics of this combustion system. This makes improvement of engine performance a difficult task. The main objective of the present study is to reveal the combustion phenomena and ignition characteristics by visualizing the combustion chamber while the engine is operated under the dual-combustion mode and the pure diesel mode at the same amount of light diesel-oil.
An open-chamber type single cylinder research engine is employed, together with an endoscope suitable for observing a small space. Endoscope was inserted into the combustion chamber to allow continuous observation of the very fast combustion phenomenon that was photographed using a high-speed video camera.
From the experimental results, ignition of pilot fuel was found to occur near the nozzle tip and, the location of the pilot flame was fixed at the nozzle tip by a later stage of combustion duration. And this pilot flame, possessing a 60-70J higher energy level than SI ignition energy, contributes to improve combustion stability in an ultra lean mixture with a lambda of approximately 2.0. Furthermore, it is proved that ignition delay of the dual condition become longer than that of diesel condition at the same injection timing. A partial pressure of oxygen lowers due to including flammable gas in pre-mixture compared with diesel condition. So this makes the reaction pilot diesel oil with oxygen inhibit under the dual condition. So it further delays ignition timing in the dual mode than that occurring in the diesel mode.