Oxidation characteristics of gasoline direct-injection (GDI) engine soot: Catalytic effects of ash and modified kinetic correlation

Abstract

In this paper, experimental analyses are conducted into the GDI soot oxidation characteristics as dependent on engine operating conditions. Soot is sampled at various engine operating conditions of a commercial 2.4 L GDI engine with a naturally aspirated, homogeneous, and stoichiometric operation strategy. The oxidation reactivity, ash composition, and carbon nanostructure of the GDI soot samples are analyzed using thermogravimetric analysis (TGA), scanning electron microscope-energy-dispersive spectroscopy (SEM-EDS), high-resolution transmission electron microscopy (HR-TEM), and Raman spectroscopy. Based on the analyses, a global GDI soot oxidation mechanism is proposed which includes the effects of soluble organic fractions (SOF)/weakly bonded carbon (WBC), and three types of ash on GDI soot oxidation. The results show that GDI soot contains an order of magnitude higher ash fraction than does conventional diesel soot, and oxidation reactivity is significantly enhanced by the catalytic effects of ash, as a function of ash content in soot. A modified empirical kinetic correlation for GDI soot oxidation is suggested on the basis of the results, and the modified kinetic correlation predicts the GDI soot oxidation rate accurately for various engine operation points at wide ranges of soot conversion and temperature without modifying kinetic parameters. The kinetic parameters are determined from isothermal and non-isothermal thremogravimetric analysis (TGA) soot oxidation tests; the methods are elucidated in detail.

Lube oil-dependent ash chemistry on soot oxidation reactivity in a gasoline direct-injection engine

“…Also, soot oxidation reactivity was in good agreement with ash fraction in soot as described in 90 % soot oxidation time (T90) over ash fraction ( Fig. 2c), which validated our previous result [4]. This result may imply that low engine-out soot mass emissions from GDI engines contribute to enhanced soot oxidation due to high ash fraction in soot.…”

“…Previously the Ca-enhancing effect on soot oxidation was also observed [5,6,9,10], so this Ca-dominant ash formed during the combustion process seems to play a similar catalytic role in soot oxidation. This promoting effect of Ca-derived ash is assumed to be comparable to fuel-borne catalysts which lower oxidation temperatures [11,12]: our previous work from [4] indicates that oxidation temperatures appreciably decreased when ash content was high. Interestingly, the activation energy of soot oxidation was invariant with ash present, like Ce-additives [11].…”

“…In our previous study [4], oxidation characteristics of GDI soot were examined at different engine operating conditions, and found that the metallic ash components in soot derived from lube oil, whose major components are Ca, P, Zn, and Na, strongly promoted soot oxidation.…”

Oxidation-derived maturing process of soot, dependent on O2–NO2 mixtures and temperatures

“…Although this surrogate soot has low contents of volatile compounds and ash [17,18], compared with engine soot, our other studies showed that the former has crystalline structure almost identical to GDI soot. Its partially oxidized samples were prepared in a thermogravimetric analyzer (TGA) as below.…”

Rußreaktivität bei Nutzfahrzeug-Dieselmotoren

“…Aus der Literatur ist bekannt, dass das Vorhandensein geringer Mengen von Metalloxiden oder nichtoxidischen Mineralien die Rußoxidation katalysieren können [11], [19]. XPS-Untersuchungen von Ascherückständen der untersuchten Pulverproben zeigen unter anderem signifikante Anteile von Na, Ca, Zn, Si, die hautpsächlich von Schmieröl-Additiven stammen.…”