Automotive spark-ignited direct-injection gasoline engines

F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 479
Fig. 39. Computed vaporization history of the injected fuel normalized by the total amount of fuel injected for an injection timing of 120 ATDC
[158].
that a tumble flow field is likely to deflect the spray towards
the cylinder wall for the centrally mounted injector position,
and it was noted that a swirl flow concentrated the spray
plume at the center of the cylinder and is effective in
reducing cylinder wall wetting. The effects of the spray
cone angle and spray-tip velocity on the lean limit were
also investigated. It was found that the lean limit could be
extended markedly when the spray-tip velocity is reduced
and the spray cone angle becomes narrower. For a solidcone
spray with a spray angle of 45, the test engine could be
operated at an overall air–fuel ratio of 40.
Optimizing the spray cone angle and tip penetration is one
of the most important steps in minimizing fuel impingement
for GDI systems other than wall-guided. With substantial
fuel impingement on the combustion chamber surfaces,
improved fuel atomization can only partially enhance the
mixture preparation. Pool-burning of the wall film will
occur, along with the associated negative effects of
increased heat loss and UBHC emissions. Dodge [54,55]
calculated the fuel wall impingement that is associated
with droplet penetration in GDI engines by computing the
drag coefficients of the droplets. This penetration distance
for impact was 20 mm for late injection versus 80 mm for
early injection. For the case of early injection of a spray
having a SMD of 15 mm, it was found that most of the
droplets decelerate to a very low air velocity prior to reaching
the piston crown. Similar results were also found for late
injection, in spite of the significantly reduced penetration
distance that is available before the spray impacts the piston.
The rapid droplet deceleration is due mainly to the higher air
densities that are encountered for the late injection condition,
which results in increased droplet drag and enhanced
vaporization rates. The worst case that was analyzed was for
a spray from a pressure-swirl atomizer with a cone angle of
52, which was found to maximize the penetration distance
while preserving spray impact on the piston crown and
cylinder wall.
The general relationship between the initial spray-tip
velocity and the spray SMD is shown in Fig. 40 for GDI
injectors and fuel pressure levels tested by Fraidl et al. [57].
It is evident from the figure that there is a limitation on the
spray-atomization benefit that can be obtained by increasing
the spray tip velocity. Any further increase in the initial
spray-tip velocity may aggravate the problem of excessive
spray penetration, but may not significantly enhance atomization.
Currently, most fuel systems for GDI engines utilize
fuel pressures in the range of 5.0–7.5 MPa, although there
are some systems that use fuel pressures of 10–13 MPa. The
curve shows that optimum atomization is associated with
initial spray-tip velocities that are in the range of 40–
50 m/s, which is approximately twice the typical peak
flow velocity of the in-cylinder flow field. Depending
upon the orientation of the spray axis relative to the incylinder
flow field, the momentum of the fuel spray can
be a substantial addition to the momentum of the flow
field. As the clearance height is generally quite small in
GDI engines, the spray of swirl injectors could result in
some degree of wall impingement even though the initial
spray-tip velocity is significantly less than is obtained with a
diesel system.
Lake et al. [148] obtained numerical predictions of the
details of fuel–air mixing inside the cylinder of a GDI
engine with a side-mounted injector. For early injection,
fuel was introduced into the cylinder between 170 and
190 ATDC on intake, whereas the corresponding values
for late injection were 20–40 BTDC on compression. It