Analysis of PDA measurements in double injection GDI sprays

Abstract

A N-heptane spray from a GDI multi-hole injector operated in ambient air at fixed conditions and with double injection commands is studied with different experimental techniques to better understand the spray behaviors, focusing the analysis on the effect of different dwell times between the two pulses. Results from spray photographic analysis, fuel injected quantity, droplet velocity and sizing by Phase Doppler Anemometry are presented and compared. The peculiarities and usefulness of a complementary application of the different techniques is illustrated. The two spray pulses have the same time length, so that the first spray evolves in a nearly quiescent and clean ambient, while the second, nominally identical to the first one, evolves in its trailing edge. The direct comparison allows an immediate perception of the differences among the two sprays, at the different dwell times, where the shorter tested, 160 microseconds, was chosen as the one that shows the first appreciable effect with at least one of the used techniques; the differences are clearly evident in the PDA results, sufficiently visible from the injection rate, not appreciable in the imaging at short distance. The effect of the longer dwell times becomes more evident and is illustrated. Keywords GDI, Spray, Double Injection, PDA, droplet sizing IntroductionThe process of fuel injection is crucial for the mixture preparation in automotive engines, and the use of fuel direct injection is considered also in gasoline engine an efficient way to attain better performances in terms of energy efficiency and pollution reduction. The experimental effort to characterize the GDI engine with its component has been huge in the last decades. The multi-hole injectors geometry and the electronic control allow to use a large number of parameters to attain the desired results, among which injector hole sizes and spray pattern, fuel pressure, injection timing and duration, where also split or multiple injections can be used [3,8]. Different experimental techniques are continuously improved and used to characterize GDI fuel sprays, both with the aim of understanding the effect of the controlling parameters, and to provide accurate data for modeling and CFD tune-up and calibration. Being split injections and GDI spray droplet sizing [5] widely studied since years, the present work accurately reports a parametric study focused on the effect of the dwell time on the spray droplet velocity and size and generally on the second spray behavior.