Variations of fuel spray characteristics during cyclic operation is directly correlated to efficient operation of an engine. Measurements of drop size distribution and drop concentration under transient conditions during injection are difficult. A very rapid and synchronized measurement technique is necessary for sectional and temporal analysis of an intermittent spray during approximately 50 ms after each injection. A pulsed-spray sizer based on Fraunhofer diffraction pattern analysis was modified so that the repeatability of each injection can be determined by an obscuration-trace measurement. The sizer with a built-in adjustable delay was synchronized to the needle-lift of the injector and the drop size data were captured over 20 microseconds. For every single injection, the attenuation of the transmitted beam monitoring the arrival, the duration, and the drop concentration in the spray was recorded and stored on a digital scope. A solenoid controlled the position of the rack on the fuel line and ensured single injection. The obscuration-trace was used as a “fingerprint” in testing the reproducibility of successive injections. Drop size measurements were statistically averaged over many reproducible cycles.

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