Modern high pressure diesel fuel injection equipment (FIE) is designed to operate
with high quality fuel that is free of external contaminants. Undissolved
bubbles of air that are normally managed by the low pressure fuel delivery
circuit, may be present for a variety of reasons. Any bubbles that persist will
violently implode following entry to the high pressure system. The effects of
bubble collapse under conditions close to atmospheric pressure are well
documented as cavitation collapse. The objective of the present paper is to
study the implosion of air/vapor bubbles in diesel fuel when exposed to much
higher pressures under controlled conditions resembling those in modern FIE. The
results demonstrate that the adiabatic temperature rise is sufficient to
initiate combustion, causing visible light emission, damage to nearby materials
and formation of black carbonaceous precipitates in the fuel. Similar black
precipitates have been previously reported in the field.
Real time sensors indicate that micro-diesel combustion of air bubbles in fuel
has several distinct phases, which appear broadly analogous to those experienced
by fuel-droplets in air in a conventional engine. These include compression,
ignition delay, combustion and finally quenching as the applied pressure and
resulting temperature are reduced. Two distinct combustion phases were observed:
a bright but brief initial phase followed by a more extended period of less
intense light emission. It is suggested that these are due to the auto-ignition
of premixed vapor, followed by a more gradual diffusion controlled flame
respectively. The effects of micro-diesel combustion were eliminated below a
critical air volume, most likely due to the entrained air going into solution
prior to attaining the minimum pressure required for auto-ignition, highlighting
the importance of effective air management in real applications.