A morphological study of soot from a diesel engine fuelled with diesel and biodiesel

Soot particulates were collected by a thermophoretic sampling technique from a single cylinder, direct injection diesel engine fuelled with diesel and biodiesel oil. The morphology of the collected samples was analysed using a transmission electron microscope and subsequent data acquisition/image processing software. Results showed the obtained biodiesel soot had smaller primary particles and aggregates as compared with those of diesel soot, due primarily to the enhanced soot oxidation by the presence of extra oxygen in biodiesel oil molecules. Introduction: Soot particulates produced during the combustion process in diesel engines have become a great concern due to their severe impacts on human health and the environment [1]. The particulates are mainly composed by soot aggregates, which are irregular clusters formed by numerous spherical-shaped primary particles. Studies on the morphologies of soot aggregates and primary particles can provide important information for improved understanding of the complexity of the soot formation and oxidation, as well as an assessment of their toxicity, chemical and deposition characteristics [2]. For measuring the particulate size, commonly used instruments such as mobility particle sizers need to dilute the exhaust gas and classify the particulates by their aerodynamic diameters. Critical issues may arise as the particle size can be influenced by the dilution ratio. Meanwhile, these instruments are not able to measure the primary particles but only aggregates [3, 4]. The current study employed a direct thermophoretic sampling technique which collects soot particles directly from undiluted exhaust flow to a grid that is suitable for the following transmission electron microscopy (TEM) analysis. The morphological characteristics of the soot particles emitted from the combustion of diesel and biodisel were compared under a fixed engine operational condition. Experimental: A standard No.2 diesel fuel (Caltex Australia Ltd.) and a biodiesel fuel derived from vegetable oil (Bioworks Australia Ltd.) were used in this study. The experiments were conducted on a 0.211L, four stroke, single cylinder, direct injection diesel engine which was coupled to a Zöllner A-100 electric dynamometer for load control. The thermophoretic sampling technique [5] was employed for collecting soot particulates from the hot exhaust stream by a thermophoretic force. In a typical experiment, a sampling line with a stainless steel stop-valve was installed along the exhaust pipe through which a grid holder with TEM copper grid attached was inserted parallel to the exhaust flow for rapid collection of soot particulates. The engine was set at the speed of 3200 rpm and 75% load condition. Soot samples collected from diesel and biodiesel combustion were examined using a high-resolution JEOL 2100 TEM at the accelerating voltage of 120kV.The TEM micrographs obtained were analysed with Image J software to reveal the soot morphological features such as primary particle diameter and distribution, aggregate area and aggregate fractal dimensions. Detailed descriptions of these analytical techniques can be found in Ref. [2, 3]. Results and Discussion: Typical features of soot particles from the combustion of diesel and biodiesel were observed, as represented in Fig.1 (a) and (b). Biodiesel soot aggregates appeared to be smaller in size and composed by relatively smaller primary soot spherules, compared with diesel soot aggregates. High-resolution TEM results shown in Fig.1 (c) and (d) indicated that both soot particles had graphite crystalline structures but with different ring orders, this may be explained by the differences on the degrees of 407 1 Conference Proceedings APMC 10 / ICONN 2012 / ACMM 22, 6-9 Feb., 2012, Perth, Western Australia