Automotive spark-ignited direct-injection gasoline engines

F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 483
droplet. A three-regime method of penetration and velocity
correlation was found to be necessary for accurate predictions;
an early time regime that is determined by the injector
opening characteristics, a middle-time region that is
controlled by spray-tip drag, and a late-time region that is
dominated by the in-cylinder flow field.
In achieving a stable stratified mixture, the important
considerations are the combustion chamber geometry, the
airflow field, and the shortening of the interval between
injection and ignition. Tomoda et al. [60] investigated the
possibility of creating a stable stratified charge using two
high-flow-rate fuel injectors; a hole-type nozzle and a swirl
nozzle, each with a piezoelectric actuator with the capability
of rapidly opening and closing the needle valve at fuel pressure
levels exceeding 15 MPa. The concept of this limiting
case is to control the degree of fuel vaporization from the
liquid film on the piston by means of the hole nozzle, and to
control the fraction of fuel vaporization in the airflow field
by means of the swirl nozzle. The injectors selected had the
capability of injecting fuel at very high pressures
(20 MPa) in a short duration (0.25 ms) to permit the
flow rates to be set independently. It was found that the
hole nozzle produces a spray with a narrow cone and a
high penetration, thus it is suitable to achieving a stratified
mixture around the spark plug at light load. As the fuel
vaporization rate achieved using the hole nozzle depends
chiefly upon heat transfer from the piston, the delivered
droplet size distribution was found to be of less importance
than that of the swirl nozzle. Visualization of the spray
development shows that vaporization of the fuel spray
from the swirl nozzle starts before any wall impingement
occurs. For this nozzle it was found that the vaporization
rate does not substantially depend on heat transfer from the
heated wall. For the hole nozzle, the vaporization rate is
suddenly enhanced as the fuel spray impacts the wall.
Therefore, an effective utilization of the thermal energy
of the piston is very important if a hole nozzle is to be
used to achieve a stratified mixture. For the low-load,
low-speed condition, the two nozzle types exhibit opposite
trends of BSFC variation with the fuel injection
pressure. For the hole nozzle the BSFC was found to
increase with a decreased fuel pressure, whereas the
swirl nozzle was found to yield an improvement in
BSFC as the fuel pressure is reduced. This trend continued
up to a limiting value of 8 MPa. For the mediumload,
high-speed condition, both nozzles require a high
fuel injection pressure to maintain low BSFC. It may be
assumed that if the injection-to-ignition interval is
decreased, the fuel pressure must be elevated in order
to shorten the injection duration. In general, since the fuel
spray of the hole nozzle has less entrainment and dispersion
than that of the swirl nozzle, a suitable stratified mixture can
be more readily achieved at light load. At higher loads, a
suitable stratified mixture could be achieved most readily
with a swirl nozzle, due to its higher rate of spray dispersion.
A study on the effect of the mean swirl ratio on the BSFC
indicates that, at light load, the BSFC is enhanced as the
swirl ratio increases.
Witaker et al. [165] studied the effect of fuel spray characteristics
on engine emissions. It was reported that spray
characteristics are of particular importance for homogeneous
operation. A comparison of the effects of spray characteristics
on engine UBHC emissions during early injection
was made between a solid-cone spray (A) and a hollow-cone
spray (B). The measured spray characteristics and the resultant
UBHC emissions for these two injectors are tabulated in
Fig. 45. It was found that the use of a hollow-cone spray
yields a decrease in UBHC emissions as the injection timing
is varied from 20 ATDC to 150 ATDC on intake. At a
timing of 20 ATDC, the measured UBHC difference
between the solid-cone and hollow-cone injectors is quite
small, but becomes significant at later injection timings. For
the solid-cone spray, the UBHC emissions increase linearly
as the injection timing occurs later during the intake stroke.
For the hollow-cone injector, however, the UBHC decreases
as the injection timing is retarded within the experimental
range. The results were derived from the same engine setup,
with only the injector changed.
The effect of fuel volatility on spray characteristics was
investigated in detail by VanDerWege and coworkers [166–
168]. It was found that the cylinder head temperature significantly
affects the resulting spray structure. The spray changed
from hollow to solid cone when the head temperature
was increased from 30 to 90C. Three types of fuel were
tested under different in-cylinder pressures. It was found
that at high temperature and low pressure, an acetonedoped
isooctane spray experienced flash boiling which
caused the spray to change from the hollow-cone structure
observed under cold conditions to a solid-cone distribution.
The transition in apparent structure takes place around 70C.
As the ambient pressure increases, the transition into a solid
cone is less pronounced; at 0.6 bar, an intermediate change
was observed. This change was not observed at higher pressures,
under which flash boiling was considered not likely to
occur, or without the low-boiling-point acetone dopant.
Experiments with indolene showed results similar to that
of the acetone-doped isooctane. This indicates that the
light components in the indolene vaporize in the hightemperature,
low-pressure case. The observed solid-cone
structure was explained by fast evaporation and flash boiling
of the volatile species, followed by entrainment of the smaller
droplets into the center of the jet, along with the induced
airflow. It is interesting to notice the marked effect of head
temperature on the spray characteristics. It is well known
that an elevated head operating temperature tends to
promote injector deposits. The amount of fuel delivered
by the injector will, in general, also vary with the increased
injector body temperature. Thus, the standard test specification
for the thermal flow shift of an injector must also be
considered. Koike et al. [169,324] reported that a high fuel
injection pressure is effective in enhancing the fuel evaporation
at low ambient temperature and pressure, and is also