Most of the new Diesel combustion concepts are mainly based on reducing local combustion temperatures and enhancing the fuel/air mixing with the aim of simultaneously reducing soot and NOx emissions. In this framework, Premixed Charge Compression Ignition (PCCI) has revealed as one of the best options to combine both low emissions and good combustion controllability.During last years, PCCI strategy has been widely explored using high EGR levels and different early or late injection timings to extend the ignition delay. Recently, the use of lower cetane fuels is under investigation. Despite the great quantity of research work performed, there are still some aspects related to PCCI combustion that are not completely well known.In this paper an experimental and numerical study is carried out focused on understanding the mixing and auto-ignition processes in PCCI combustion conditions using Diesel and Gasoline fuels.For this purpose, a parametrical study has been performed varying EGR, injection timing, and fuel type in a High Speed Direct Injection (HSDI) Diesel engine. A detailed analysis in terms of air/fuel mixing process has been also developed by means of a 1-D in-house spray model.Results using Diesel fuel show how increasing the ignition delay by advancing the injection timing together with high EGR levels extends the mixing time between the End of Injection (EoI) and the Start of Combustion (SoC). As a consequence, lower local equivalence ratios are achieved. However, it has been confirmed how the use of gasoline has more impact on air/fuel mixing process and auto-ignition delay than any other engine setting investigated with the diesel fuel.