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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.

Nowadays HCCI combustion process is revealing the most useful technique for reducing pollutant emission from internal combustion engines. In the present paper, HCCI combustion was realized by means of single late injection at high pressure and heavy EGR, up to 50%. A transparent Direct Injection (DI) diesel engine equipped with high pressure Common Rail (CR) injection system was used. The engine was fed with commercial diesel fuel and ran in continuous mode. Digital imaging and spectroscopic techniques, with high temporal and spatial resolution, were applied to study the low temperature combustion process. Injection and combustion phases were analysed by digital imaging. Mixing process, autoignition and pollutants formation were investigated by Broadband Ultraviolet - Visible Extinction Spectroscopy (BUVES) and flame emission measurements. Radicals and species such as OH, CH and CO were detected in the combustion chamber.

Spectroscopic measurement and high speed visualization were used in single cylinder, four-stroke DI diesel engine, optically accessible. It was equipped with a four valves head and fully flexible electronic controlled ‘Common Rail’ injection system. The effect of pilot and main injection on combustion process was evaluated. Mixing formation, autoignition and soot formation process were analyzed by broadband ultraviolet-visible flame emission spectroscopy and high-speed digital imaging. The autoignition phase occurred near the tip of the jet and was characterized by strong presence of OH radicals for both investigated conditions The presence of C2 and OH radicals strongly characterized CR diesel combustion process during soot formation and evolution. In particular, high presence of OH concentration for the whole process from the autoignition to the soot formation and successive phases contributes to lower soot levels.