As global environmental issues are becoming even more critical, there is further increasing pressure on the automotive industry to develop engines with reduced emissions and better fuel economy while meeting performance requirements. One of the developments in automotive engine technology to meet these requirement is the Gasoline Direct Injection (GDI) system. Introduction of fuel at elevated temperature is a method employed which can cause the fuel to undergo flash boiling within the nozzle hole, and thereby inducing enhanced atomization. The aim of the present work is to investigate the flash boiling mechanism inside fuel injector nozzles using the advanced Hertz-Knudsen model implemented in a commercial CFD code. The model is first validated by a benchmark nozzle test case where the investigation is conducted to interpret various aspects of bubble number density and its variation with respect to the degree of superheat. The obtained results from the simulation and the one from the benchmark test case show a substantiated comparison. In the second part of this work, the flash boiling model is tested in a realistic 8-hole GDI Injector, from Engine Combustion Network database, with an attached high-pressure gas chamber. The Mass, momentum and enthalpy equations were solved for a 3-phase system (liquid fuel, fuel vapour and air) with applied interfacial exchange models between them.