Due to stringent emission standards, the demand for higher efficiency engines has been unprecedentedly high in recent years. Among several existing combustion modes, pre-chamber spark ignition (PCSI) emerges to be a potential candidate for high-efficiency engines. Research on the pre-chamber concept exhibit higher indicated efficiency through lean limit extension while maintaining the combustion stability. In this study, a unique pre-chamber geometry was tested in a single-cylinder heavy-duty engine at low load lean conditions. The geometry features a narrow throat, which was designed to be packaged inside a commercial diesel injector pocket. The pre-chamber was fueled with methane while the main chamber was supplied with an ethanol/air mixture. The ‘avalanche activated combustion’ or L.A.G. process was explored which relies on enriched pre-chamber combustion to generate radicals which, upon being discharged into the main combustion chamber, will trigger ignition sites distributed in the combustion chamber, thus achieving fast combustion. The ability of PCSI concept to enhance the lean limit with progressive enrichment in the pre-chamber was demonstrated. In addition, passive pre-chamber concept, where no additional fuel was injected into pre-chambers, was also explored and compared against the fueled pre-chamber experiments. The processed data features fast combustion rates with high combustion stability with the evident extension of the lean combustion limit. The engine-out emissions, measured by the exhaust gas analyzer, were reported together with the combustion data.