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Soot Volume Fraction Measurements in a Gasoline Direct Injection Engine by Combined Laser Induced Incandescence and Laser Extinction Method
Abstract:
In order to study the soot formation and oxidation phenomena
during the combustion process of Gasoline Direct Injection (GDI)
engines, soot volume fraction measurements were performed in an
optical GDI engine by combined Laser-Induced Incandescence (LII)
and Laser Extinction Method (LEM). The coupling of these two
diagnostics takes advantages of their complementary
characteristics. LII provides a two-dimensional image of the soot
distribution while LEM is used to calibrate the LII image in order
to obtain soot volume fraction fields. The LII diagnostic was
performed through a quartz cylinder liner in order to obtain a
vertical plane of soot concentration distribution. LEM was
simultaneously performed along a line of sight that was coplanar
with the LII plane, in order to carry out quantitative measurements
of path-length-averaged soot volume fraction. The LII images were
calibrated along the same path as that of the LEM measurement. A
calibration factor was obtained for each pair of LII and LEM
measurements. The analysis of the evolution of this calibration
factor for varying measurement conditions was used to evaluate the
validity of the methodology.
These techniques were then applied to study the in-cylinder soot
concentration evolution in a GDI engine equipped with an
outward-opening piezoelectric injector. The impact of injection
strategy and EGR rate on soot concentration were studied in
stratified operation. Results show that soot production is strongly
correlated to hot fuel-rich regions. In competition with the soot
production phenomena, soot oxidation processes are responsible for
a dramatic decrease (1 to 2 orders of magnitude) of in-cylinder
soot volume fraction during the expansion stroke until about 90 CAD
after the combustion TDC. Soot oxidation is mainly governed by the
mixing between soot and hydroxyl radicals. Beyond ~90 CAD after TDC
in-cylinder mean temperature seems to be too low for soot-oxidation
reactions to occur. Additional measurements were performed under
homogeneous fuel rich conditions in order to further validate the
techniques. The soot volume fraction evolution showed that no soot
oxidation occurred. This result is consistent with the fact that
almost no oxidant is left after the main combustion process under
these conditions.