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Transient Liquid Penetration of Early-Injection Diesel Sprays
Abstract:
Diesel low-temperature combustion strategies often rely on early injection timing to allow sufficient fuel-ambient mixing to avoid NOx and soot-forming combustion. However, these early injection timings permit the spray to penetrate into a low ambient temperature and density environment where vaporization is poor and liquid impingement upon the cylinder liner and piston bowl are more likely to occur. The objective of this study is to measure the transient liquid and vapor penetration at early-injection conditions. High-speed Mie-scatter and shadowgraph imaging are employed in an optically accessible chamber with a free path of 100 mm prior to wall impingement and using a single-spray injector. The ambient temperature and density within the chamber are well-controlled (uniform) and selected to simulate in-cylinder conditions when injection occurs at -40 crank-angle degrees (CAD) or fewer before top-dead center (TDC). Injector parameters such as injection pressure, injection duration, nozzle orifice size, multiple injections, and fuel distillation properties are also varied.
Results show that an injection duration less than one-half of the development time for a steady liquid length will produce liquid penetration distances that are less than that of a steady-state spray. Using multiple, short injections also limits the liquid penetration while permitting the vapor-phase to continue to penetrate into the chamber. Small nozzle orifice diameters are helpful in limiting liquid penetration if sprays reach a steady-state liquid length. However, if the total injected mass is kept constant, requiring a longer injection duration with a small orifice diameter, the maximum liquid penetration is the same as that for a larger orifice diameter (provided injection durations are short enough such that liquid does not reach the steady-state liquid length). Similar findings are also obtained with injection pressure variation and the same injected mass: short injection durations at high injection pressure produce the same maximum liquid penetration as longer injection durations at low injection pressure. Fuel distillation property variation shows an expected benefit of shorter liquid penetration for low-boiling-point-temperature fuels.