The classic model of fuel combustion in diesel engines is incorrect according to researchers at Sandia National Laboratories (Livermore, CA). The group is using a specially modified diesel engine and a combination of planar laser Rayleigh-scatter imaging and planar laser-induced incandescence (LII) imaging to study diesel fuel-jet penetration, vaporization, and mixing-process dynamics as well as relative soot concentrations and particle-size distributions in the combusting fuel jet.
Prior to the testing program, diesel-engine designers thought injected liquid-phase fuel penetrated far into the combustion chamber and that both liquid-phase and vapor-phase diesel fuel were present in the combustion zone. They also believed that the combusting fuel jet consisted of a pure-fuel core with a diffusion flame around the periphery and that soot particles occurred on the fuel-rich side of the diffusion flame in a shell around the periphery.
Laser imaging tests demonstrate, however, that, under typical conditions, all the liquid diesel fuel vaporizes within 25 mm of the injector and that the fuel in the main combustion zone is in the vapor phase. Quantitative fuel-vapor-concentration measurements show that, just downstream of the maximum liquid penetration, the fuel vapor is uniformly mixed with air.
Imaging of soot formation indicates that the soot actually occurs throughout the cross section of the combusting fuel jet, starting just downstream of the liquid-fuel region. Both the concentration and particle size of the soot increase toward the leading edge of the jet, with the highest concentrations and largest soot particles occurring at the front of the jet (see Fig. 1). Clusters of large soot particles from this region persist late into the combustion event, which means they might never oxidize and may eventually come out of the engine exhaust pipe.
