In modern Internal Combustion (IC) engines, the fuel spray atomization process is known to play a key role in affecting mixture formation, combustion efficiency and soot emissions. Therefore, a thorough understanding of the fuel spray characteristics and atomization process is of great importance. In this study, the fuel spray of modern Gasoline Direct Injection (GDI) engines and diesel engines has been experimentally and numerically studied. At the same time, optimized physical-numerical spray breakup models for the spray simulation have been developed and validated.