Spray auto-ignition is a complex physical and chemical process whose mechanism is still not well understood. This paper explores the contribution of physical and chemical mechanisms to spray auto-ignition and combustion behaviors over a wide range of ambient temperature and pressure conditions in an optical rapid compression machine. Specifically, the spray development and ignition process are first visualized and the spray ignition delay times (IDTI) are measured through high-speed imaging. IDTI is then compared with gas phase chemical ignition delay times (IDTC) calculated by 0D homogeneous reactor simulation. Subsequently, different combustion modes are recognized by analyzing the mixture status at the instant of ignition, the spray flame behavior, and the pressure evolution history. Finally, a regime diagram of combustion modes is proposed to illustrate the dominant mechanisms for different spray combustion modes. Results show that the measured spray IDTI is longer than the 0D calculated IDTC due to the physical delay caused by spray development, evaporation, and mixing. At higher temperatures and pressures, the difference between IDTI and IDTC is increased because the evaporation and mixing become progressively important, compared to the chemical reaction mechanism. Scrutinization on the pressure and the apparent heat release rate evolution curve reveals that with the increase of the temperature and pressure, the chemical-controlled combustion time accounts for less and less of the total combustion duration. This further indicates that spray ignition and combustion behaviors transit from chemical-dominated mode to mixing-dominated mode.