KEYWORDS: GDI; Pressure-swirl; Multi-hole; Penetration length; Cone angle;
SMD; Equivalence ratio; Split injection; Spray-wall interaction.

The Gasoline-direct injection (GDI) engines have received significant research atten-
tion worldwide over the last few decades. This is due to their ability to combine the

benefits of direct fuel injection (such as higher volumetric efficiency, reduced knock-
ing tendency, lower emissions etc.) into the cylinder as in diesel engines, with reduced

NOx and particulate matter emissions similar to Port-fuel injected (PFI) spark-ignition
engines. In GDI engines, the fuel is directly injected into the engine cylinder, in short

pulses at high injection pressure. However, issues such as wall wetting, mixture inho-
mogeneity, spark-plug fouling etc. are major hurdles in realizing the benefits of GDI.

In addition, advent of new strategies such as split injection are useful for preparing
the fuel mixture for part load (stratified mixture) or full load (homogeneous mixture)

engine conditions. Although split injection strategies are well understood for diesel en-
gine (CI) operating conditions, the associated spray dynamics and mixture preparation

for volatile fuels such as gasoline/iso-octane are not well understood. Hence the present

work focuses on the study of pulsatile spray dynamics and wall-interaction in GDI in-
jectors under elevated ambient pressure and/or temperature conditions relevant to real

engine operating conditions.
The main objective is to understand the influence of high ambient pressure and
temperature on unsteady spray dynamics and evaporation, for the sprays of single-hole
pressure swirl as well as multi-hole injectors for volatile fuels. A combined numerical
and experimental approach is adopted. The spray simulations have been performed
using ANSYS FLUENT based on a two-way coupled, Eulerian-Lagrangian method to
predict the gas phase using unsteady RANS and droplets using Discrete phase model
(DPM). Iso-octane was used as the fuel. The primary and secondary breakup along
with droplet collisions are suitably modelled. Spray characteristics including spray

v

structure and penetration length, SMD and mixture distribution are calculated. The
predicted local mean drop sizes and spray penetration length are in agreement with the

experimental results (Han et al., 1997; Kay et al., 2012). The predicted results indi-
cate that the spray is narrower and penetrates less at higher ambient pressure. In this

respect, the additional forces on droplets due to inward radial pressure gradient as well
as increased drag are examined in detail. The effect of ambient conditions on the spray
evaporation process is studied based on the spatio-temporal evolution of the mixture
equivalence ratio.

Next, the evolution of pulsatile and split injection spray from a multi-hole injec-
tor at different ambient conditions is examined experimentally and computationally for

gasoline application. The influence of different split proportions on the spray charac-
teristics such as penetration length, cone angle and SMD is characterized in detail.

The results from the split injection cases are compared against those of single injec-
tion cases to highlight the differences. Experiments have been conducted in a constant

volume chamber for elevated ambient pressures, and time-resolved spray images are

recorded using high-speed shadowgraphy technique. Good agreement between the ex-
periments and simulations is achieved with respect to spray penetration length and cone

angles for both injection strategies. It is observed that orienting the grid along the spray
axes improves the accuracy of prediction by reducing artificial diffusion effects. As
the proportion of the first injection is increased, keeping the injection duration same,
the interaction of the second spray with the rear of the first spray is evident, from the
spray images at higher chamber pressure. The penetration length is found to reduce for
split injection strategy at all ambient pressure conditions. The numerically predicted
spatio-temporal evolution of vapour plumes in terms of equivalence ratio highlights the

possibility of attaining near homogeneous, rich mixture or a stratified mixture depend-
ing on the split proportion.

A preliminary experimental investigation on spray-wall interaction for high wall
temperature conditions is also presented, for the multi-hole injector. The experiments
are performed under atmospheric conditions. The temporal evolution of post-impingement
spray characteristics including spray width (SW ) and height (SH) is studied for different

wall temperatures and stand-off distances. Small vortex-like structures are identified af-
vi

ter spray impingement and the differences due to wall temperature and wall stand-off

height are highlighted. A strong effect of droplet evaporation under high wall temper-
ature condition is observed with a reduction of spray width and height at later time

instants. The effect of split injection strategy on the spray dispersion characteristics
after impingement is also examined.