Study of the Mixing and Combustion Processes of Consecutive Short Double Diesel Injections

The mixing and combustion processes of short double Diesel injections are investigated by optical diagnostics. A single hole Common Rail Diesel injector allowing high injection pressure up to 120MPa is used. The spray is observed in a high pressure, high temperature cell that reproduces the thermodynamic conditions which exist in the combustion chamber of a Diesel engine during injection. Three configurations are studied: a single short injection serving as a reference case and two double short injections with short and long dwell time (time between the injections). Several optical diagnostics were performed successively. The mixing process is studied by normalized Laser Induced Exciplex Fluorescence giving access to the vapor fuel concentration fields. In addition, the flow fields both inside and outside the jets are characterized by Particle Imaging Velocimetry. The combustion process is studied by simultaneous 355 Laser Induced Fluorescence (LIF) and OH LIF, providing simultaneous information about the localization of formaldehyde, poly-aromatic hydrocarbons (PAH) and hydroxides. Finally, soot concentration is measured by the Laser Extinction Method. The analysis of the mixing processes shows that, in the case of a short dwell time, the velocity fields associated with each injection interact, leading to an increase of the mixing rate at the head of the second injection. This interaction and the resulting effects on mixing are reduced if the dwell time is increased. The analysis of the combustion processes shows that in the case of double injections, the ignition of the second injection is promoted by the entrainment of high temperature gases originating from the combustion of the first injection. As a consequence, while for the conditions studied no soot are formed during the combustion of the first injection, the combustion of the second injection is more fuel rich and therefore forms PAH and soot. Also, for the operating conditions studied, it was found that when the dwell time is increased, the extension of the combustion of the first injection leads to the presence of low temperature intermediates closer to the injector nozzle at the timing of the second injection. As a result, those higher temperature gases are entrained faster in the second jet, provoking faster ignition, and stabilization of the flame closer to the nozzle. Consequently the combustion is more fuel-rich, therefore forming higher concentrations of PAH and soot compared to the shorter dwell time.