Liquid and Vapor Phase Distributions in a Piloted Diesel Fuel Spray 950445
An optically-accessible single-cylinder compression-ignition two-stroke research engine equipped with dual-injection system, image acquisition, and control system have been designed to acquire two-dimensional images of the pilot and main diesel fuel sprays. The engine construction permits illumination of the sprays by a thin sheet of laser light from a pulsed Nd:YAG laser frequency tripled to operate at the ultraviolet wavelength of 355 nm. The liquid fuel was decane with TMPD-naphthalene dopant dispersed in it. Upon ultraviolet excitation by the pulsed laser, liquid fuel regions fluoresced with a spectrum centered at the wavelength of 380 nm, while vapor regions fluoresced with a spectrum centered at 470 nm. This approach, called Exciplex technique, was applied to permit simultaneous acquisition of the liquid and vapor fuel regions in the cup-in- head geometry of the combustion chamber. The results from limited conditions suggest that existence of the pilot injection affects the chamber flow field, particularly in the near-wall region, in such a way that less liquid fuel accumulation occurs with pilot injection scheme thus should reduce hydrocarbon emission. The area spreading rate of both vapor and liquid phases within the visualization plane were linear in early stages with former having a higher rate. Close to the impingement time, vapor phase area penetration is reduced to near zero while liquid continues linearly but at slower rate than its initial value. Both air utilization and evaporization are therefore reduced. Also, based on this and works by others, suggestion is made that to achieve optimum simultaneous reduction in soot and NOx, the timing between the pilot and main injections (dwell) should be chosen with due considerations of the interaction between the swirl flow and pilot spray and the relative magnitudes of the ignition delay period with respect to the dwell.