Automotive spark-ignited direct-injection gasoline engines

468
F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562
Fig. 29. Schematic and spray characteristics of the air-assisted injector
[72]: (a) schematic of the injector design; (b) injected air-to-fuel
ratio as a function of the engine load; and (c) effect of the air rail
pressure on droplet size.
overall mean drop size. However, it is generally agreed that
it is more difficult to develop a portfolio of off-axis, angled
sprays with this constraint. The off-axis spray is one of the
developments that address the packaging of injection
systems. Component packaging in GDI design is a primary
constraint, and the movement from a 12 mm to a 10 mm to
an 8 mm injector mounting diameter is being driven by the
need to enhance packaging options. The availability of
injectors that deliver fuel sprays that are directed 10, 20
or 30 from the injector mounting axis can offer significant
flexibility in the original design configuration of a GDI
combustion chamber.
The high-pressure, single-fluid, swirl-channel injector
using an inwardly opening pintle is by far the most common
GDI injector type, and there are intense programs by injector
manufacturers to continue to improve injector and spray
performance. The non-spray performance parameters of the
injector include the opening time, closing time, pintle
bounce, durability, dynamic range, noise level, power
consumption, leakage and operating pressure range. The
spray performance parameters include the mean diameters
of the delivered main spray and the sac spray, as well as the
corresponding values for DV90. Other key spray parameters
include the cone angles of the spray (both the initial angle
and the final collapsed spray angle), the spray-deviation
(skew) angles of the main and sac-volume sprays, the sac
and main tip penetration rates and velocities, the drippage,
after-injections or ligament formation upon injector closure
and the fuel mass distribution within the spray. Additional
measures of performance, both spray and nonspray, are
related to injection-to-injection and unit-to-unit variability
in all of the above parameters.
With such rapid development occurring in GDI injector
design and performance, it is obvious that a compilation of
best performance values for each of the above parameters,
which constitutes a table of best practice performance, is a
living document. The injector performance envelope is
continually being pushed as new and improved injectors
are designed, built and tested. Table 1 contains a tabulation
of current best practice for a number of key injector performance
parameters. As injectors have a range of design
fuel rail pressures, four categories are listed, with each category
having current best-practice values for the performance
metrics. All of the droplet size statistics presented in the
table were obtained by a two-component, phase Doppler,
real-time-system analyzer for a very wide range of injector
hardwares. All injectors were measured for indolene fuel at
20C using a standard test protocol.
2.3.6. Future requirements of GDI fuel sprays
Research by a number of workers in Japan, Europe
and the United States indicates that a working design
goal for current GDI fuel injection systems should be to
achieve a symmetric fuel spray having a SMD of less
than 20 mm, with 90% of the injected fuel volume
(DV90) in drops smaller than 40 mm, while requiring
fuel pressures that are on the order of 5.0–7.5 MPa.
This fuel pressure range will generally provide a longer
pump life than is obtained using 8.0–13 MPa, with less
pump noise and a pressure rise to a greater percentage of