Automotive spark-ignited direct-injection gasoline engines

F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562 519
residual unburned mixture from the first cycle instead of
residual burned mixture. The IMEP produced is therefore
greater than that obtained in any later cycle in spite of the
fact that the combustion chamber surfaces are coldest on the
first firing cycle. The lack of combustion on the first cycle
with the GDI engine, however, does result in elevated
UBHC emissions as shown by the fast FID measurements
in Fig. 84(b). By the fifth cycle, the UBHC emissions attain
a steady-state level, whereas the UBHC level builds steadily
in the PFI engine until the first fire in the seventh cycle. The
pattern of high UBHC emissions continues through as many
as 35 cycles as misfire or partial burns continue for the PFI
engine.
Takagi [24] reported results from a cold starting test of a
1.8L, four-cylinder GDI engine. As shown in Fig. 85, it was
found that the engine exhibited improved transient response
and a significant reduction in UBHC emissions when
compared with a standard PFI engine. Four cycles were
required for the GDI engine to attain a stable IMEP when
operating at an air–fuel ratio of 14.5, whereas the PFI
engine required 12 cycles to reach stable operation at an
air–fuel ratio of 13. A comparison of the engine emission
characteristics of GDI and PFI engines during a cold start
was made by Shimotani et al. [182] for a coolant temperature
range of 20–23C, using both early intake fuel injection
(30 ATDC on intake), and near bottom dead center injection
(170 ATDC on intake). The UBHC emissions for the
PFI and GDI engines for early injection were found to be
higher than that obtained for the GDI engine using injection
at the start of the compression stroke [367]. The effect of
injection timing on the transient UBHC emissions during the
first several seconds of a cold start was also conducted. The
results of this study show that the UBHC emissions
decreased and reached a minimum at 170 ATDC on intake,
which is a 60% reduction when compared with that of early
injection (30 ATDC on intake). The effect of coolant
temperature on the part-load UBHC emissions and fuel
consumption was a third investigation. The engine was
operated at an engine speed of 1400 rpm with a BMEP of
150 kPa. The UBHC emissions were observed to decrease
as the injection timing was retarded from early in the intake
stroke to the middle of the intake stroke, and to increase for
further later timings. At a coolant temperature of 20C for
the GDI engine, a 50% reduction in the steady-state UBHC
emissions was obtained by retarding the injection timing
from 30 ATDC to 110 ATDC on intake. For a coolant
temperature of 80C, the minimum BSFC occurred at an
injection timing of 110 ATDC on intake. The data indicated
that the injection timing must be optimally set at 110
ATDC on intake for cold operation in order to obtain low
UBHC emissions, whereas a timing of 30 ATDC on intake
is optimum for warm, steady-state operation. This timing
schedule provides a good balance between UBHC emissions
and fuel consumption. The effect of the operating air–fuel
ratio on the cold start performance of GDI and PFI engines
was also measured. Compared to the PFI engine, the GDI
engine can operate two full ratios leaner at the same starting
cycle number.
A vehicle test of an Isuzu prototype GDI engine was
conducted by Shimotani et al. [182]. The injection timing
was set at the end of the intake stroke for cranking, and at
110 ATDC on intake following the first firing. A stoichiometric
air–fuel ratio was used. The UBHC and CO emissions
upstream of the catalysts were measured for the first
cycle in the LA-4 mode. The emissions data that were
obtained during the first 60 s after the cold start are shown
in Fig. 86(a). Through an optimized injection timing and the
use of a leaner mixture, the GDI engine exhibits a clear
reduction in both UBHC and CO. The catalyst temperature,
UBHC conversion efficiency, and the integrated UBHC
concentration after the catalyst are shown in Fig. 86(b),
which is for the same engine operating condition as in
Fig. 86(a). It was noted that the GDI engine provides a
more rapid catalyst response, and that the UBHC emission
level is lower than that obtained with the PFI engine. This is
due to the rapid increase of the catalyst temperature after a
cold start, resulting from the more rapid firing and improved
A–F control capability afforded by direct injection. The
time history of the air–fuel ratio from the 11th cycle of
the LA-4 mode is shown in Fig. 86(c). For the conventional
PFI engine, numerous calibration compensations are
required to minimize the effect of fuel transport lag caused
by the inherent fuel–wall wetting [327]. During speed and
load transients the effectiveness of such compensation is
limited, and tip-in and tip-out spikes of UBHC emissions
occur. The more accurate control of the transient air–fuel
ratio for the GDI engine is illustrated in Fig. 86(c), and the
associated catalyst efficiency is shown in Fig. 86(d). It may
be seen that the PFI engine shows a large fluctuation in the
catalyst efficiency during engine transients, indicating a
decrease in the UBHC conversion efficiency. This was not
observed during the operation of the GDI engine, which
implies that the more precise air–fuel control of the GDI
engine can yield an associated improvement in the catalyst
conversion efficiency. As a result, the UBHC emissions can
be reduced, even during steady-state operation. Both the
emissions and the BSFC are improved during the entire
LA-4 mode for the GDI engine. The total reduction of the
UBHC emissions can reach 45% for the entire LA-4 mode,
showing the potential of the GDI engine for meeting the
future emission standards for UBHC.
As opposed to UBHC emissions during a cold-start, the
UBHC emissions for low-load operation represent a significant
problem that is historically associated with gasoline
DISC engines [223]. In the case of gasoline direct injection,
the flame is eventually quenched in the extra-lean area at the
outer boundary of the stratified region where the mixture is
in transition from rich to lean [24]. As a result, a significant
amount of UBHC remains in the total charge. The inability
of the flame front to propagate from the rich mixture at the
spark gap through all of the lean mixture in the outer portion
of the combustion chamber is a key factor contributing to