Although low diesel fuel prices have reduced the appeal of natural gas (NG) engines recently, the CO2 advantage and low NOX and PM potential of NG makes it well-suited for meeting future greenhouse gas (GHG) and potential lower NOX regulations for on-road medium and heavy-duty engines. However, traditional NG fueling strategies and/or poor air/fuel ratio control can result in significant levels of tailpipe methane (CH4) emissions which offset the CO2 advantage due to the high global warming potential of CH4. 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 shown to improve the brake thermal and combustion efficiencies over equivalent fumigated dual-fuel combustion modes in a previous publication. In addition, DI2 operation was shown to deliver a two point (% fuel energy) brake thermal efficiency (BTE) improvement compared to the baseline diffusion-controlled combustion strategy (HPDI) where the NG injection occurs after the diesel injection. Details of the DI2 combustion were analyzed using 3D-CFD and suggested that further improvements of the combustion system were possible. Specifically, CFD indicated that reductions in unburned HCs from the crevice regions may be possible by reducing the included spray angle of the NG nozzle. The spray angles of the baseline injector are fairly wide since HPDI operation is intended for near Top-Dead-Center (TDC) injection timings and not for earlier injection timings as required for DI2. Therefore, modified injection nozzles with narrow NG spray angles were procured to evaluate the potential HC reduction on the engine. This paper reports the results obtained with the modified nozzles which confirmed that significant HC emission reductions of up to 40% can be achieved while maintaining the high BTE of the DI2 combustion mode.