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 initiated combustion 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, different pre-chamber geometries were tested in a single-cylinder heavy-duty engine at different loads. The geometries were prepared with three different pre-chamber volumes and with three varying nozzle area to pre-chamber volume ratios. The pre-chambers were fueled with methane while two sets of experiments were conducted, the first with ethanol as main chamber fuel and the second with methane. 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 volumetric heat release. Parametric studies were performed with different pre-chamber fueling rate. 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 study was coupled with CFD simulations to explain the phenomenon of mixture formation and flow characteristics in the pre-chamber. 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.