Micro-Diesel Combustion of Entrained Air in High Pressure Fuel Injection Systems 2018-01-5019

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.