Lean combustion is one of the most promising methods for increasing engine efficiency and reducing the exhaust emissions from S.I engines. A disadvantage of lean operation, however, is that the burning rate of mixtures is reduced or combustion becomes partial and, sometimes, outright misfiring. Stable flame kemel formation and fast burn are needed to guarantee a successful subsequent combustion under lean-burn operation. High energy ignition system (HEIS) with prechamber was desinged to extend the lean misfire limit and also to improve burning rate of lean mixture. It was tested on a visualized sigle-shot, single-cylinder engine. A shadowgraph method was adopted to visualize the early combustion process. The performances of HEIS were compared with those of normal spark ignition case where the ignition position was at half-radius.
Contrary to previous engine test results by others, there was no extension in the lean misfire limit with current homogeneous prechamber combustion. The major difference between the two seemed to be caused by that in the chamber temperatures. However, the stratified prechamber combustion extended lean misfire limit up to $\varphi$ = 0.55 whereas the limit with conventional spark ignition at half-radius was $\varphi$ = 0.66 at 1500 rpm.
