Split injection in a homogeneous stratified gasoline direct injection engine for high combustion efficiency and low pollutants emission

The effects of splitting the injection event in a GDI (gasoline direct injection) engine operating with a HOS (homogeneous stratified) lean charge are analysed through experimental and numerical techniques. Injection is assumed as divided in two parts, each delivering the same gasoline amount, the first occurring during intake, the second during compression. The work is initially focused on the experimental characterization of the engine under study for the collection of data concerning the in-chamber combustion development. Beside measurements of in-cylinder pressure, UV chemiluminescence is applied to follow the OH radicals formation from spark ignition up to the late combustion phase, thanks to the optical accessibility to the combustion chamber. The collected data serve to the validation of a properly formulated three-dimensional (3D) CFDs model for the simulation of the whole engine working cycle. The definition of the control strategy leading to the greatest combustion efficiency and lowest pollutants emission is made in two steps through numerical optimization: the 3D CFD model is first run to build a low number of samples to be used within a Gaussian RSM (response surface method) to reconstruct, in the DOE (design of experiments) space, the integral of the in-cylinder pressure over volume in the closed valve period; in the second step, the coupling between the 3D engine model and the Simplex algorithm is performed in a restricted DOE subdomain defined according to the first step analysis. The assessed methodology highlights the synchronization of both spark ignition and split injection leading to the highest power output and the lowest pollutants release. The experimental verification of the numerical findings is finally carried out.