Spray penetration and mixture formation in GDI engines are
crucial to a reliable ignition and the subsequent combustion. For
the prediction of the mixture formation process, computational
fluid dynamic simulations are applied. Therefore, details about the
nozzle exit conditions are essential, either as boundary conditions
to be set, or to validate the numerical results.
This paper presents experimental results on the influence of
boundary conditions on the spray structure at the nozzle exit of a
GDI injector. The injector investigated is an outwardly opening
piezo injector, generating a hollow cone spray with a string
structure. The distribution of the strings (the so-called
"string structure") is needed for the starting conditions
of the computational fluid dynamic simulations, as the origin of
the strings is unresolved so far. The experiments are conducted in
a constant volume pressure chamber varying the ambient temperature
between 300 and 600 K and the ambient pressure between 0.1 and 1.0
MPa. Analyzing the impact of the different injection conditions,
the injection pressure is varied between 8 and 12 MPa, utilizing
energizing durations between 200 and 600 μs. In addition, the
needle lifts are varied, too. Visualization techniques are used to
investigate the spray structure and the penetration length.
The evaluation of the results is divided into two parts. First,
spray images of the nozzle exit flow are compared concentrating on
general parameters of the spray such as the penetration length. In
the second part, the string structure is investigated by extracting
the light intensity profile at certain nozzle distances. The
analysis of the spray structure shows that the ambient and
injection pressure as well as the ambient temperature influence the
circumferential fuel distribution at the nozzle exit; the needle
lift shows no significant influence in the visualization results.
Comparing the influence of the different parameters on the string
distribution shows that the injection pressure and the ambient
temperature have the greatest influence. But even though the
greatest influence can be observed varying the injection pressure
and the ambient temperature, these influences are very small.