Automotive spark-ignited direct-injection gasoline engines

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F. Zhao et al. / Progress in Energy and Combustion Science 25 (1999) 437–562
diameters of less than 10 mm. The new EPA “fine particle”,
or PM2.5, standard includes all particles having equivalent
diameters of less than 2.5 mm. This indicates the heightened
interest in smaller particles that is reflected in the evolution
of PM emissions standards. For particulates from diesel
engines, only particles in the size range above 100 nm in
diameter were found to have any significant effect on the
mass-weighted PM10 and PM2.5 mean values, and nuclei
mode particles were found to have little or no effect on
the mass-weighted distribution regardless of the number
density [233].
Historically, gasoline engines have been exempt from the
requirement to meet the particulate emissions standard for
diesel engines. The justification for this has been that gasoline
engines produced particulate emissions that were on the
order of only 1% of those diesel engines. This was certainly
the case prior to the advent of recent diesel particulate legislation
[235]. Recent studies indicate that current gasoline SI
engines often emit an increased fraction of nanoparticles
even though steady-state particulate number emissions are
generally several orders of magnitude lower than those from
modern diesel engines [234,236]. Particulate number
emissions from gasoline engines have also been shown to
increase significantly when operated under high-load, transient
and cold-start conditions. It was reported by Graskow
et al. [236] that, unlike the PM emissions from diesel
engines, the particulate emissions from PFI engines are
quite unstable. Typically a stable baseline concentration of
engine-out PM emissions is on the order of × 10 5 particles/
cm 3 ; however, a “spike” in the PM emissions is occasionally
observed. These spikes are found to be composed of nearly
100% volatile particles of less than 30 nm in diameter, and
can exhibit number densities exceeding 100 times that of the
baseline concentration. An analysis of particulates from PFI
engines by Andrews et al. [235] revealed that the bulk of the
mass is ash, with the second largest fraction being unburned
lubricating oil. Carbon emissions were found to be significant
only at high load with mixture enrichment, whereas, at
other operating conditions, carbon was reported to comprise
less than 10% of the total PM mass. The large ash fraction of
the gasoline PM emissions were found to include a large
fraction of metal compounds, with calcium and sodium
evident for operation at low load without EGR, and copper
and magnesium predominant for operation with EGR.
Preliminary research indicate that GDI engines, as evolving
powerplants for automotive applications, may emit a
larger amount of particulates than do conventional PFI
engines, especially during stratified-charge operation.
Depending on the degree of combustion system optimization,
smoke emissions from prototype GDI engines could be
as high as 1.2 BSU [100,101,210]. A comparison of the
particulate emissions for a current PFI SI engine, a current
production GDI engine and a 1995 European IDI diesel
engine for the US FTP cycle is illustrated in Fig. 94(a)
These data represent mass measurements of particulate
matter collected on filter media. It may be seen that the
level of PM emissions for the GDI engine is between
those of the diesel and PFI SI engines. As reported by
Maricq et al. [237], the PM emissions from a vehicle
powered by a GDI engine are on the order of 10 mg/mile.
In comparison, the PM emissions from a comparable dieselpowered
vehicle are on the order of 100 mg/mile. As
compared to the 1–3 mg/mile PM emissions from a model
PFI engine [235,237], the PM emissions from current
production GDI engines are relatively high, even though
they are well below the current US standard of 80 mg/
mile as measured on the FTP test cycle. In interpreting
this published comparison, it should be noted that both the
PFI and GDI engines have been developed and massproduced
without major specific efforts being directed
towards the minimization of particulate emissions.
Graskow et al. [238] measured the particulate emissions
from a 1998 Mitsubishi GDI engine using a chassis-dynamometer
test. It was reported that the average polydisperse
number concentration was on the order of × 10 8 particles/
cm 3 and that the number-weighted, geometric mean particle
diameter was from 68 to 88 nm. In contrast, modern PFI
engines tested by the same authors [234,236] emit average
particulate number concentrations ranging from 10 5 particles/cm
3 at light load to 10 7 particles/cm 3 at high load.
Older PFI engines have been shown to emit number concentrations
in excess of 10 8 particles/cm 3 for conditions corresponding
to highway cruise operations. It was also reported
that, for the operating conditions tested, the numberweighted,
geometric mean diameters of the particulate
matter emitted from the GDI engine are larger as compared
to the PFI engines tested. This relatively large mean particle
size is likely to increase the particulate mass emissions from
the GDI engine, but could also lead to a decrease in the
relative fraction of particles emitted in the nanoparticle
size range.
Maricq et al. [237] investigated the PM emissions from a
1.83L, four-cylinder, four-valve production GDI engine for
a range of operating conditions. As illustrated in Fig. 94(b),
the particle number emissions are found to increase by a
factor of 10–40 when the operating mode of the GDI engine
is stratified-charge instead of homogenous-charge. The
emissions of particulate matter exhibit a strong dependence
on the GDI injection timing, and it was observed that the
particle number and volume concentrations increase markedly
as the injection timing is retarded. Advancing the spark
timing generally yields an increase in both the particle
number concentration and the mean particle size for both
homogeneous and stratified-charge operation. An increase
in the engine speed and load generally leads to an increase in
PM emissions; however, this trend is dependent on the injection
timing for stratified-charge operation. Based upon a
chassis-dynamometer test that emulated the FTP test
cycle, it was found that the PM emissions exhibit very
substantial fluctuations, with a strong correlation existing
between vehicle acceleration and the observed increases in
PM emissions. These increases are theorized to occur