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Rohani, Behzad,Bae, Choongsik Elsevier 2017 Applied thermal engineering Vol.116 No.-
<P><B>Abstract</B></P> <P>The physio-chemical characteristics of soot particles are of importance with regard to performance of diesel after-treatment systems. In this study, the soot particles generated in a single-cylinder heavy-duty diesel engine are examined in terms of nanostructure, oxidative reactivity and volatile organic fraction (VOF), using thermogravimetric analysis (TGA), X-ray diffraction (XRD), Raman micro-spectroscopy, and high resolution transmission electron microscopy (HRTEM). Five different injection strategies including single injection and multiple injections with various pilot injection amounts and dwell times were tested with and without exhaust gas recirculation (EGR), while combustion phasing, engine speed, and fuel injection quantity was matched for all cases. Results indicate that for the soot produced under EGR condition, nano-structural order (indicated by crystallite size obtained from XRD and AD1/AG resulted from the Raman Analysis) can explain the soot reactivity. However, in the absence of EGR, the reactivity trend cannot be explained by the structural order. It is discussed that a possible reason can be a higher level of in-cylinder oxidation in non-EGR cases (indicated by higher level of surface functional groups) which roughens the soot surface, and enhances the oxidation by increasing the specific soot surface area. It is also found that in the absence of EGR, different injection strategies impact the soot reactivity and VOF content, which can be explained mainly through the level of charge premixed-ness and the in-cylinder soot oxidation, and the consequent effect on the soot surface roughness. Results also show that the influence of injection strategy on the soot reactivity and VOF content tends to vanish as EGR is added.</P> <P><B>Highlights</B></P> <P> <UL> <LI> EGR reduced the nano-structural order, regardless of injection strategy. </LI> <LI> EGR reduces both VOF and reactivity, regardless of injection strategy. </LI> <LI> Longer dwell time between pilot and main injection increases VOF and reactivity. </LI> <LI> With EGR, VOF and reactivity are both reduced and un-affected by injection strategy. </LI> <LI> VOF-reactivity correlation (without causality) suggests role of surface roughness. </LI> </UL> </P>
Morphology and nano-structure of soot in diesel spray and in engine exhaust
Rohani, Behzad,Bae, Choongsik Elsevier 2017 Fuel Vol.203 No.-
<P><B>Abstract</B></P> <P>This work investigates how morphology and nano-structure of the soot particles produced in a diesel spray flame evolve due to the rise in the temperature/pressure caused by the piston motion. The soot particles were sampled from the exhaust line of a diesel engine and were compared to the soot particles directly sampled from the spray flame of the same injector, in a constant volume chamber. Analysis of the high resolution transmission electron microscope (HRTEM) images acquired from the soot samples at two axial locations in the spray flame shows that the soot aggregates grow in size, become longer, more fractal and attain lower surface to volume ratio along the spray axis. However, the soot particles produced in the spray flame become smaller, shorter, less fractal, more compact and attain higher surface to volume ratio when emitted from the engine. This is discussed to be due to the increase in the oxidation rate by temperature rise caused by piston motion, and the consequent oxidation induced fragmentation of the soot aggregates. The effect of oxidation is also evident on the size of the soot primary particles, as the engine-out soot is found to have smaller primary particles compared to the soot initially produced in the spray flame. Nano-structural analysis shows that the crystallite size in the engine-out soot is increased compared to the soot originally produced in the spray flame. This suggests that among two possible effects of temperature on diesel soot crystallite size, being crystallite size increase due to enhanced graphitization from one hand, and crystallite size reduction due to oxidation enhancement from the other hand, the former effect is dominant and the soot particles become structurally more ordered under the effect of temperature rise caused by piston motion.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Soot was sampled in two locations in diesel spray flame and also at engine exhaust. </LI> <LI> Morphological and nano-structural analysis was done by TEM image processing. </LI> <LI> Soot particles become larger and more fractal along the spray axis. </LI> <LI> Engine-out particulates become smaller and less fractal compared to spray soot. </LI> <LI> Engine-out particulates become nano-structurally more ordered compared to spray soot. </LI> </UL> </P>