Homogeneous Charge Compression Ignition (HCCI) combustion was applied to a transparent diesel engine equipped with high pressure Common Rail (CR) injection system. By means of CR system the quantity of fuel was split into five injections per cycle.
Combined measurements, based on digital imaging and spectroscopic techniques, were applied to follow the evolution of HCCI combustion process with high temporal and spatial resolution. Digital imaging allowed to analyse injection and combustion phases. Broadband ultraviolet - visible extinction spectroscopy (BUVES) and flame emission measurements were carried out to evaluate the presence of radicals and species such as HCO, OH, CH, and CO. In particular, BUVES measurements were performed to follow fuel oxidation, and pollutant formation and oxidation.
During injection and cool combustion, bands of aromatic compounds and alkyl peroxides, indicating fuel decomposition, and hydrogen peroxides were detected. Autoignition was characterized by the presence of OH radical which was homogenously distributed in the chamber and provided for the reduction of particulate matter in the cylinder. Finally, OH could be considered as a marker of HCCI combustion and it could result a suitable tool to identify the different phases of combustion. The effect of different injection pressures was analysed.