Direct injection (DI) diesel engine is one of the most promising economic engines with low fuel consumption being developed. However, combustion characteristics of DI diesel engine should be controlled carefully to meet the emission regulations, which are becoming more and more stringent. The spray characteristics in DI diesel engine are so crucial for combustion that advanced fuel injection systems, which give higher injection pressure and more controllability, have been developed recently. Common-rail injection system is a newly developed fuel injection system, which can supply injection pressure up to 1350bar regardless of engine speed and load condition.
There have been various efforts to understand the nature of diesel spray. Nevertheless, the details of transient diesel spray formation are not certain yet. This uncertainty mainly arises from the difficulty of optical access to the dense region of the spray near the hole and lack of information about internal flow conditions. The temporally and spatially resolved microscopic visualization near the nozzle hole can offer not only validation data for two-phase internal flow model of the nozzle, but also clues to understand the transient diesel spray formation mechanism. The structures of transient diesel spray have been assumed from the indirect methods or extrapolative experimental results at dilute regions of the spray. Assorted experimental correlations on behavior of transient diesel spray are available on literatures. Most of the works were done under quasi-steady injection condition. For high-speed direct injection (HSDI) diesel engine, however, injection rate shape is triangular or trapezoidal; quasi-steady assumption is not valid. The effect of injection rate change on spray characteristics during injection period should be understood to develop universal correlations applicable to diesel sprays from different injection systems.
In this study, non-evaporating diesel sprays from conventional mechanical pump injection system and newly developed common-rail injection systems were characterized. Macroscopic spray parameters including penetration and spray angle were obtained from spray images with the purpose-built image processing software. Injection rate meter based on hydraulic pulse principle was designed and measured injection rates were correlated with spray characteristics. Spray front was accelerating at initial period of injection due to injection rate increase. The duration of this period was proportional to characteristic time scale $(t^*)$. As enough time elapsed from start of injection, spray penetration increased according to the square root of time similar to fully developed turbulent jet. Spray angle was found to have negative correlation with injection rate change.
Entrainment velocity of ambient air into transient diesel spray was measure with LDV (Laser Doppler Velocimetry) technique at cylindrical control surface around the spray. Mass flow rate of ambient air into diesel spray was changed according to injection rate and entrainment coefficient increased with non-dimensional distance $(z/z^*)$ unlike the constant value of fully developed turbulent jet.
Microscopic images taken at the vicinity of the nozzle exit, where the distance from nozzle exit was shorter than so called breakup length, showed that this optically dense region was not liquid column but tangled thick ligaments or membranes. Most of the liquid droplets were formed at the tip of ligaments due to the disturbance developed on it. Some smaller liquid droplets seem to be generated from bubble or membrane breakup process. SMD (Sauter Mean Diameter) value decreased while injection rate increased and higher injection velocity resulted smaller droplets.
The results of this study reveal that injection rate increase at early stage of injection significantly affects spray characteristics including spray penetration, spray angle and entrainment of ambient gas.
