Effect of exhaust gas recirculation (EGR) and multiple injections on diesel soot nano-structure and reactivity

다국어 초록 (Multilingual Abstract)

<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>