EXPERIMENTAL-STUDY AND MODELING OF DODECANE IGNITION IN A DIESEL-ENGINE

Two experiments have been performed under conditions as close as possible to those existing in a diesel engine. The first is oxidation of n-dodecane in a motored diesel engine running under conditions close to ignition but avoiding it. The progress of chemical reactions is followed by measurements of the global temperature increase Delta T of the exhaust gases, and by continuous sampling of the combustion chamber gases, to measure the concentrations of hydroperoxides and molecular hydrogen; about 4.2% of the energy introduced as hydrocarbon is consumed, thus showing significant transformations during the ignition delay of n-dodecane. The location of the maximum concentration of hydroperoxides coincides with the fuel jet's edge. Tarlike compounds are present in the unburnt dodecane at the engine exhaust. The second experiment is the study of ignition delay of an n-dodecane spray in an oxidation chamber filled with air, between 715 and 760 K and 15 and 25 bar. A reduced mechanism of 32 reactions, with three types of branching due to the species (RO(2), RO(2)H), (HO2, H2O2), and H, enable one to predict the ignition delay. Computer simulations are made with the KIVA II code. They show good agreement between the experimental and the calculated ignition delays. They also indicate that, during the ignition delay, reactions occur first at the boundary of the fuel spray. A temperature increase of about 100 K takes place at the hottest points, which correspond to concentration maxima of the three branching species. Time-dependent evolutions of average concentrations show that RO(2)H reaches a maximum first, then H2O2, and lastly the H atom.