The present paper deals with a comprehensive analysis of the knocking phenomenon through experiments and numerical simulations. Conventional and non-conventional measurements are performed on a 4-stroke, 4-cylinder, turbocharged GDI engine. The engine exhibits optical accesses to the combustion chamber. Imaging in the UV-visible range is carried out by means of a high spatial and temporal resolution camera through an endoscopic system and a transparent window in the piston head. This last is modified to allow the view of the whole combustion chamber almost until the cylinder walls, to include the so-called eng-gas zones. Optical data are correlated to in-cylinder pressure-based indicated analyses in a cycle resolved approach.
The numerical investigation is performed through a properly developed 3D CFD model of the engine under study, that employs the Extended Coherent Flamelet Model for the combustion initiated by the spark plug and the Shell model to reproduce the low temperature chemical activity in the mixture not yet reached by the main flame front. The role of the intermediate species of the Shell model in localizing both spatially and temporally the occurrence of undesired self-ignitions is highlighted.
Numerical and experimental data well correlate, in particular regarding the self-ignition location in the combustion chamber on the side where the injector is positioned. The opposite side, in fact, is positively affected by the charge cooling effect deriving from the subtraction of the latent heat of vaporization of the spray droplets.
