To achieve higher thermal efficiency of a gasoline engine, numerical simulations were conducted. The effects of air-fuel mixture formation in partially premixed compression ignition (PaPC) as well as fuel ignitability on auto-ignition combustion under boosted and high EGR ratio air charge condition were studied. From the simulations, it was showed that EGR with boosted air charge could make the oxidation process remained in a partially oxidized state which is desired for PaPC combustion. Thermal efficiency of the PaPC engine is expected to be up to around 50% by selecting fuel ignitability, fuel injection timing and the quantity and EGR ratio.