Laser-induced multi-point ignition with single-shot laser using conical cavities and prechamber with jet holes

Laser-induced multi-point ignition techniques with a single-shot unfocused laser have been proposed and experimentally tested by utilizing conical cavities and a prechamber for methane/air mixtures. Flame propagation in a constant-volume engine-like combustion chamber was visualized with high speed shadowgraphy and the pressure history was measured. A conical cavity with an opening near the apex installed in the prechamber and a closed conical cavity in the main chamber could induce two-point laser-induced cavity ignition by the focusing effect along the centerline of the cavity by the reflection of the irradiating laser beam on the axisymmetric cavity surface, leading to a thermal breakdown. An additional ignition could be achieved relatively simultaneously by the rapid ejection of hot burnt gas through the cavity opening to the main chamber. The installation of extra two jet holes provided additional ignitions by the ejections of hot burnt gases into the main chamber, thereby maximum of five ignition sources can be utilized simultaneously in the experiment. The pressure measurements showed that these ignition techniques demonstrated rapid combustion behavior and higher maximum pressure rise, resulting in a significant reduction in the total combustion time comparable to those in practical engines.