Experimental study of the influence of the fuel and boundary conditions over the soot formation in multi-hole diesel injectors using high-speed color diffused back-illumination technique

Two prototypes multi-hole diesel injector from a high-pressure common rail system were used to operated with diesel and n-Heptane fuels in a high-pressure and high-temperature test rig able to reach 1100 K and 15 MPa under different oxygen concentrations with the goal to study the influence of the boundary conditions on the combustion process evaluating parameters as ignition delay, lift-off length and soot formation. A novel optical set-up was implemented to visualize the soot cloud evolution in the fuel jet between 30 and 85 mm from the nozzle exit using the high-speed color diffused back-illumination technique, it was used as a result of the insertion of a high-pressure window in the injector holder opposite to the frontal window of the vessel allowing an optical arrangement in line-of-sight. The experiments performed in this work used a LED with a wavelength of 460 nm to provide information about physical of the soot properties, the results of varying the operational conditions show the reduction of soot formation with an increase in injection pressure, a reduction in ambient temperature, a reduction in oxygen concentration or a reduction in ambient density. Furthermore, fuel effect on soot formation also has been studied showing a big difference between both fuels, diesel is known to be more prone to produce soot than n-Heptane reducing the ability of the technique implemented, especially in conditions with high temperature and high density.