Abstract
The effect of split injection on the mixture characteristics of DISI (Direct Injection Spark Ignition) engines was investigated firstly by the Laser Absorption Scattering (LAS) technique. Through splitting the fuel injection process, two possible benefits were found: 1) High density liquid phase spray piling up at the leading edge of the spray can be circumvented, subsequently the reduction of the spray tip penetration; 2) The quantity of "over lean" (φ_v<0.7) in the spray can be significantly reduced. These are believed to contribute to the reduction of the engine-out smoke and HC emissions. In order to clarify the mechanism behind the effect of the split injection, the spray-induced ambient air motion was investigated by the LIF-PIV technique. The strong ambient air entrainment into the tail region of the spray and a counter-vortex structure were found in both the single and split injections. In the case of the single injection, the spray developed in extending its length, subsequently a larger volume resulted and thus it was diluted to "over lean" by the air entrainment. In contrast, in the case of split injection. The second spray was injected into the tail region of the first spray and it distributed some distance from the leading edge of the first spray and the evaporation of the second spray was promoted by the ambient air motion. Hence the replenishment of the liquid phase second spray on the leading edge of the first spray would be reduced. As a consequence, the high density liquid phase spray piling up at the leading edge was avoided. In addition, the ambient air motion played a positive role on evaporating the spray into an "more combustible" (0.7<φv<l.3) mixture. This was especially true in the tail region of the spray and the region where the counter-vortex motion was occurring.
