Split injection in a GDI engine under knock conditions: an experimental and numerical investigation

Present work investigates both experimentally and numerically the benefits deriving from the use of split injections in increasing the engine power output and reducing the tendency to knock of a gasoline direct injection (GDI) engine. The here considered system is characterized by an optical access 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 placed 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 of the mixture, where undesired self-ignition may occur under some circumstances. Optical data are correlated to in-cylinder pressure oscillations on a cycle resolved basis. The numerical investigation is performed through a properly developed 3D CFD model of the engine under study, which employs a flamelet model for the combustion initiated by the spark plug, and a low-temperature self-ignition model in the zones not yet reached by the flame front. The difference in the engine behavior if powered under single or double injection strategies and their influence about knocking are discussed. Split injection reduces engine cycle-by-cycle variability with respect to the single injection case, all the others relevant parameters remaining unchanged. Benefits are also obtained as regards the resistance to knocking. This is a consequence of the different flow fields arising under the two powering modes, which obviously affect the formation of chemical intermediate species in the low temperature regimes preceding self-ignition.