Quantitative Study Of Diesel Spray Structure And Characteristics Of Twice Fuel Injection By PLIEF Technique

Low Temperature Combustion (LTC) engines are being paid much attention andwidely investigated due to their potential of high thermal efficiency and very lowemissions of NOx and particulate matter (PM). For diesel engine, fuel injection,atomization, vaporization and mixing with the air is a key step to the wholecombustion process. In this study, a constant vessel which was designed to simulatethe high temperature and pressure environment in diesel engine was developed andPlanar Laser Induced Exciplex Fluorescence system was established for theinvestigation of evaporated diesel spray. The highest1100K temperature and densityof60kg/m3environmental simulation was achieved. The concentration andtemperature field both of vapor phase and liquid phase were quantitatively studiedunder heavy-duty diesel engine similar conditions in the constant-volume bomb byusing Planar Laser Induced Fluorescence technique. On this basis, the effects ofambient density on diesel spray structure and characteristics were quantitativelystudied under single fuel injection and twice fuel injection.The development of the spray featured three stages according to the spraystructure parameters, fuel injection rate, evaporation rate and gas entrainment rate. Inthe early stage of spray development, vapor phase and liquid phase are concurrent, theevaporation rate is low, and the equivalence ratio of vapor phase spray is lower than2.In the middle stage of spray development, liquid core length increases exponentiallywith time until to its maximum value, the evaporation rate increases and the massfraction of>2and the transient maximum equivalence ratio of vapor phase spraygradually increase. In the fully developed stage of spray, liquid core length remainsbasically unchanged, vapor phase spray keeps on developing forward, the gasentrainment occupies an leading position so the mass fraction of>2of vapor phasespray decreases. A stratified distribution of concentration and temperature field wasobserved during the development of spray. The concentration both of vapor phase andliquid phase gradually increase from0to a maximum from outside to the inner, thetemperature of vapor phase spray decreases gradually from the mean environmentalvalue to the lowest from outside to the inner.In the twice fuel injection, the injection pulse width are0.6ms and1.0ms respectively. The interval of the two injection signals is1.0ms. During the dwellbetween injection pulses, the mixture becomes leaner. The mass fraction of leanmixture with <1increases and the mass fraction of rich mixture with>2decreases.It results in a low soot formation rates.Ambient density has significant effect on diesel spray breakup, atomization andpenetration. Both of vapor phase and liquid phase penetration decrease as the ambientdensity increases. The increase of ambient density promotes the development of spray,the increase of spray homogeneity in the vapor phase spray head, and incrassation ofthe sheath of vapor phase spray. The increase of ambient density promotes theevaporation of liquid drop. The earlier spray fully evaporates, the more homogeneousthe mixture is. On the other hand, it is hard to fully utilize the ambient gas with highambient density. So it is necessary to consider the matching of combustion chamberand spray penetration when the engine is with a supercharger.