Automotive spark-ignited direct-injection gasoline engines

F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 455
Fig. 15. Schematic of other GDI injector concepts: (a) two-step
nozzle [65]; and (b) PZT nozzle [22].
size distributions between swirl-type and hole-type highpressure
fuel injectors [60], it is clear that even though the
difference in the mean droplet size (SMD) between the
sprays from these two injectors is only 4 mm, the holetype
nozzle produces a wider droplet-size distribution
having many larger droplets that are theorized to be responsible
for the observed increase in the UBHC emissions. It
should be noted that the use of finer atomization may or may
not reduce the UBHC emissions, depending upon the incylinder
turbulence level, due to small pockets of very
lean fuel–air mixtures [75–77,348]. A strong turbulence
level in the combustion chamber is required to enhance
the fuel–air mixing process by eliminating small pockets
of very lean mixture. An important point to consider is
that the increased droplet drag that is associated with finer
atomization reduces the spray penetration, which can
degrade air utilization.
Even though a design fuel rail pressure of 5.0 MPa seems
Fig. 16. Schematic of the AlliedSignal high-pressure swirl injector
[83].
to be high enough for producing an acceptable GDI spray,
Xu and Markle [58] recommended a higher fuel pressure for
some of the reasons listed below. From their measurements,
the SMD of the Delphi outwardly opening GDI injector is
reduced from 15.4 to 13.6 mm as the fuel pressure is
increased from 5.0 to 10 MPa. Such a small SMD reduction
does not seem to be significant, but the total surface area for
an injected quantity of 14 mg of fuel is increased 13%,
which should lead to a direct improvement in the fuel vaporization
rate. More importantly, an elevated high injection
pressure may be required to reduce the key statistic for the
maximum droplet size of the spray, namely the DV90. It was
reported that the DV90 of the Delphi GDI injector spray at
30 mm downstream from the injector tip is reduced from 40
to 28 mm when the fuel injection pressure is increased from
5.0 to 10 MPa. In addition to its effect on spray characteristics,
an elevated fuel rail pressure may also reduce the
injector flow rate sensitivity to injector stroke variations.
2.3.2. Single-fluid high-pressure swirl injector
The development of the spray from a GDI injector may be
divided into discrete stages. The first is the initial atomization
process that occurs at or near the injector exit. This is
mainly dependent on the injector design factors such as
nozzle geometry, opening characteristics, and the fuel
pressure [275,289]. The second stage of spray development
is the atomization that occurs during the spray penetration
process, which is dominated by the interaction of the fuel
droplets with the surrounding airflow field. The spray cone
angle is an important parameter that is nominally determined
by the injector design; however, in the actual application
the spray cone angle of a swirl injector also varies
with the in-cylinder air density and, to a lesser degree, with
the fuel injection pressure [78–81]. With the pressure-swirl