The CO2 advantage coupled with the low NOX and PM potential of natural gas (NG) makes it well-suited for meeting future greenhouse gas (GHG) and NOX regulations for on-road medium and heavy-duty engines. However, because NG is mostly methane, reduced combustion efficiency associated with traditional NG fueling strategies can result in significant levels of methane emissions which offset the CO2 advantage due to reduced efficiency and the high global warming potential of methane. To address this issue, the unique co-direct injection capability of the Westport HPDI fuel system was leveraged to obtain a partially-premixed fuel charge by injecting NG during the compression stroke followed by diesel injection for ignition timing control. This combustion strategy, referred to as DI2, was found to improve thermal and combustion efficiencies over fumigated dual-fuel combustion modes. In addition, DI2 provided significant thermal efficiency improvement over the baseline diffusion-controlled combustion strategy (HPDI) where NG injection occurs after diesel injection. The DI2 combustion process was analyzed using 3D-CFD and indicated that additional CH4 reductions from the crevice region may be possible by reducing the NG nozzle spray angle. To evaluate the potential reduction available, modified injection nozzles with narrow NG spray angles were tested on the engine. This paper reports the CFD and experimental results obtained with the modified injection nozzles which confirmed that significant CH4 emission reductions can be achieved while maintaining high BTE with DI2.