The effects of reductions in the ambient gas oxygen concentration on the flame lift-off length on direct-injection (DI) diesel fuel jets under quiescent conditions were experimentally investigated. Reductions in the ambient (i.e., in-cylinder) gas oxygen concentration occur in an engine when exhaust gas recirculation is used to reduce the emission of nitrogen oxides. Also examined were the effects of the changes in lift-off length observed for various conditions on the total amount of oxygen entrained upstream of the lift-off location, soot formation, and the relationship between fuel vaporization and combustion processes. The research was conducted in a constant-volume combustion vessel using a common-rail fuel injector and a Phillips research grade #2 diesel fuel.
The lift-off length measurements show that lift-off length is inversely proportional to the ambient gas oxygen concentration. They also show that previously observed trends in lift-off length with respect to other parameters [1,2] remain the same over the range of ambient gas oxygen concentration considered. Estimates of the total amount of oxygen entrained upstream of the lift-off length show that it does not change with decreasing ambient gas oxygen concentration due to the inverse relationship between lift-off length and oxygen concentration. Furthermore, the oxygen entrainment estimates coupled with total soot incandescence measurements indicate that the link previously shown between soot formation and the amount of fuel and ambient gas that mix upstream of the lift-off length in a fuel jet  holds for reduced oxygen concentration conditions. Finally, the results show that as the ambient gas oxygen concentration decreases and the lift-off length increases, the combustion and soot formation regions in a fuel jet are pushed downstream and stretched out. This latter trend results in less interaction between fuel vaporization and combustion processes in a fuel jet as the oxygen concentration decreases.