Automotive spark-ignited direct-injection gasoline engines

F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 535
Fig. 101. Schematic of the Mitsubishi GDI combustion system [20]:
(a) fuel injection strategies; and (b) piston geometry.
representative of a fully stratified combustion system,
whereas the Ford PROCO combustion system was purposely
designed with the objective of matching the specific
output of the pre-mixed-charge engine using high EGR
rates. The PROCO stratified-charge engines employed an
injection system with an operating fuel pressure of 2 MPa,
with injection occurring early in the compression stroke,
while the TCCS engines utilized a high-pressure, dieseltype
injection system. Although the minimum BSFC values
Fig. 102. The Mitsubishi GDI engine system layout [50,51].
of both of these engines were quite good, the control of
UBHC emissions was extremely difficult for light-load
operation. NOx emissions were controlled using EGR rates
of up to 50%. Even with this level of EGR, the NO x emissions
for both the PROCO and TCCS systems significantly
exceeded the current emission standards, thus the use of a
lean-NO x catalyst would be required if these combustion
strategies were to be implemented today. The power output
of the late-injection TCCS system was also soot-limited.
Further studies of stratified-charge engines have shown
that high air swirl is desirable for obtaining the required
level of fuel–air mixing, and that high-energy ignition
sources enhance the multi-fuel capability. In spite of significant
measures to reduce autoignition and maximize air
utilization, these engines were not able to achieve the specific
power output of then-current PFI engines. Additional GDI
engine concepts from the 1970s that are based upon different
geometric combinations of fuel spray plume and spark gap,
such as the Mitsubihsi combustion process (MCP) [239], International
Harvester and White Motors (IH–White) [240], and
Curtiss–Wright stratified charge rotary combustion (SCRC)
[241] are shown in Fig. 100(a)–(c), respectively.
A direct-injection combustion system for both GDI and DI
diesel engines that operates by injecting fuel onto a central
pedestal in a piston cavity was developed by Kato et al.
[242,243]. In this combustion system, fuel is injected by a
single-hole nozzle against the flat surface of the pedestal.
The fuel deflects orthogonally and symmetrically in a disk
shape and forms the air–fuel mixture. As the result of having
a comparatively rich mixture present in the vicinity of the
pedestal, and of initiating ignition at the center of the rich
mixture, stable combustion is achieved. As the air–fuel
mixture is always formed near the pedestal, there is little
fuel in the squish area. Therefore, it is possible to prevent
end-gas detonation in GDI applications and it is claimed that
this type of engine can be operated at compression ratios of up
to 14.5 on gasoline. With this combustion system, however,
the UBHC emissions are found to be very high, which represents
a significant development problem [244].