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For the US market, an abundant supply of natural gas (NG) coupled with recent green-house gas (GHG) regulations have spurred renewed interest in dual-fuel combustion regimes. This paper explores the potential of co-direct injection to improve the efficiency and reduce the methane emissions versus equivalent fumigated dual-fuel combustion systems. Using the Westport HPDI engine as the experimental test platform, the paper reports the results obtained using both diffusion controlled (HPDI) combustion strategy as well as a partially-premixed combustion strategy (DI2). The DI2 combustion strategy shows good promise, as it has been found to improve the engine efficiency by over two brake thermal efficiency (BTE) points (% fuel energy) compared to the diffusion controlled combustion strategy (HPDI) while at the same time reducing the engine-out methane emissions by 75% compared to an equivalent fumigated dual-fuel combustion system.