| Two-dimensional laser-sheet imaging and high-speed cinematography have been used to examine the fuel-air mixing and combustion process in a newly constructed, optically accessible, direct-injection diesel engine (bore {dollar}times{dollar} stroke: 140 {dollar}times{dollar} 152 mm) at 1200 rpm and motored TDC conditions of 1000 K and 5 MPa.; At first Mie-scatter images of the fuel droplets show that the liquid fuel penetrates almost linearly with increasing crank angle until reaching a maximum length of 24 mm approximately 3{dollar}spcirc{dollar} after the start of injection (ASI). Then the liquid length remains fairly constant as the vapor-fuel jet continues to penetrate across the chamber. High resolution elastic scatter experiments verify that beyond 27 mm from the injector nozzle all the fuel is entirely vapor-phase. In this vapor-phase region, which corresponds to the main combustion zone in this engine, planar laser Rayleigh scattering was applied to obtain quantitative two-dimensional fuel-vapor concentration images of the fuel-air mixture in the engine at fired and non-fired operating conditions. Additionally, fuel-vapor concentration images were further reduced to equivalence ratio images using an adiabatic mixing approximation to model the local temperature of the evaporating Diesel jet.; The images show that at 4.5{dollar}spcirc{dollar} after the start of injection, which corresponds to the time just before the first indicated heat release, the fuel and air are relatively well mixed in the leading portion of the Diesel jet with equivalence ratios mostly between 2 and 4 and temperatures as low as 700 K (ambient air temperature 1000 K). The edges of the jet are well defined with the signal level rising sharply from the background level up to levels corresponding to equivalence ratios in the jet. After 5.0{dollar}spcirc{dollar} ASI, in the non-reacting jet, the overall equivalence ratio in the front of the jet decreases due to continuous fuel-air mixing and more regions have a fuel-air mixture of stochiometric value. Finally, comparisons of Rayleigh images of the reacting and non-reacting jet show that the initial breakdown of the fuel, indicated by a significant decrease in the Rayleigh signal intensity, occurs at 5.0{dollar}spcirc{dollar} ASI throughout the cross-section of the leading portion of the Diesel jet. |
