The current trend of downsizing used in gasoline engines, while reducing fuel consumption and CO2 emissions, imposes severe thermal loads leading to the possible auto-ignition (AI) of fresh gases hot spots around Top-Dead-Center (TDC). Temperature plays a major role for knock and heat transfer between the solid and the fluid highly dictates the in-cylinder temperature levels around TDC, due to a higher surface to volume ratio. This paper presents a comprehensive numerical methodology for an accurately prediction of thermodynamic conditions inside the combustion chamber based on Conjugate Heat Transfer (CHT). The fundamental bricks for the simulation are well described: internal aerodynamics, spray calibration and combustion model. Finally, conjugate heat transfer is solved by means of a coupled simulation between the fluid and the solid cylinder-head and valves. Wall heat transfer are well captured and used to solve for the solid. The resulting surface temperature in-homogeneities are used as boundary condition to solve for the fluid. The CHT methodology is successfully applied to the Renault H5Ft downsized direct injection engine operating at 2500 rpm. The simulation results are compared to a classical RANS simulation where Dirichlet boundary conditions are imposed for the wall temperatures. Experimental in-cylinder pressures and temperature distribution on the cylinder dome are used for validation. Tentative of knocking approach based on CHT coupling with detailed chemistry in fresh gas is done.