Automotive spark-ignited direct-injection gasoline engines

F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 539
Fig. 107. Combustion chamber configuration of the Toyota GDI
engine [41].
• The crossover point of the European version from lean
(40) to normal (14.5) air–fuel ratio was increased from
2000 to 3000 rpm. The lean range vehicle speed is
extended to highway cruise at 120–130 km/h.
• The intake manifold was extended from 265 to 400 mm
to enhance the low-speed torque characteristics.
• A two-stage injection strategy, called two-stage mixing,
was added to improve low-speed torque and suppress
knock by injecting part of the fuel during intake stroke
and the remaining portion during compression.
• A second two-stage injection strategy, called two-stage
combustion, was employed during cold start for a more
rapid catalyst light off by injecting a fraction of the fuel
during the intake stroke and the remaining portion during
expansion stroke.
The largest GDI engine so far developed by Mitsubishi is a
4.5L, 90, V8 engine using 32 valves and four camshafts.
The piston bowl is reported to be a slightly shallower than
that in the original I4 and V6 GDI engines. Two highpressure,
single-plunger, injection pumps are utilized, one
for each cylinder bank.
6.3. Toyota combustion system
The Toyota direct-injection combustion system has been
described in several recent publications [21,22,41,43,60,
139,170,203,226,255–261,273,319,356,358,359,361,371].
The principal components and stratification concepts that
are used in the GDI engine, which is designated as the D-4,
are illustrated in Figs. 106–108. As shown in Fig. 107, a
uniquely shaped cavity in the piston is employed. Zone (A)
of the cavity is designed to be the mixture formation area,
and is positioned upstream of the spark plug. The wider
zone (B) is designed to be combustion space and is effective
in promoting rapid mixing. The increased width in the
swirl flow direction was reported to enhance the flame
propagation after the stratified mixture is ignited. The
involute shape (C) is designed to direct the vaporized
fuel towards the spark plug. The intake system consists
of both a helical port and a straight port, which are fully
independent. An electronically activated SCV of the
butterfly-type is located upstream of the straight port.
When the SCV is closed, the resulting swirl ratio is
reported to be 2.1. The helical intake port utilizes a variable-valve-timing-intelligent
(VVT-i) cam-phasing system
on the intake camshaft. These valves are driven by a DC
motor so that the desired valve opening angle can be
controlled according to the engine operating conditions.
Fig. 109 shows the detailed SCV operating map. For
light-load operation the SCV is closed, which forces the
induction air to enter through the helical port, thus creating
a swirling flow. Highly atomized fuel is injected into the
swirling flow near the end of the compression stroke. The
flow moves the rich mixture to the center of the chamber
around the spark plug, while part of the fuel disperses into
the air in the combustion chamber, forming a stratified air–
fuel mixture. For heavy-load operation the SCV is opened,
and the intake air is inducted into the cylinder with a lower
pressure loss. The fuel is injected during the intake stroke,
resulting in a homogeneous mixture. The main features of
this GDI system are summarized as follows:
• four cylinders;
• four-valve pentroof combustion chamber;
• bore × stroke—86 × 86 mm;
• displacement—1998 cm 3 ;
• compression ratio—10:1;