An Investigation of Diesel PM Particle Morphology Using TEM and SEM

Abstract

This research investigates the effect of engine load and combustion mode on the size, shape and microstructure of PM particles of two diesel engines using a scanning mobility particle sizer (SMPS), and electron microscopes including scanning electron microscope (SEM) and transmission electron microscope (TEM). The effect of engine load on the morphology of PM particle was investigated using a 2004 MACK MP7-355E heavy-duty diesel engine. The effect of combustion mode on the morphology of the PM particles was preliminarily examined using 2005 Opel 1.9 L light-duty diesel engine. The SEM images were processed using ImageJ V: 1.44o software to obtain a set of parameters based on the projected area. These include the maximum length, maximum width, project area equivalent diameter, shape descriptor, fractal dimension, and the size of primary PM particles. The TEM images were processed to obtain the micro-structure and chemical composition of PM particles. The SEM images obtained in this research revealed the diverse morphology of PM particles formed in heavy-duty diesel engine. The operation at low load tends to form small sphere-like PM particles compared to medium and high load operation featured with large and chain-like PM particles. The projected area equivalent diameter of PM particles observed was in the range of 43.7 to 167.4 nm and 66.4 to 688 nm for operation at 20% and 100% load respectively. The PM particles smaller than 43 nm observed using SMPS were not observed using SEM methods. The projected area equivalent diameter of PM particles from the light duty engine during PCCI mode was smaller in size compared to the PM particles observed under normal operation. The fractal dimension of this heavy-duty diesel engine observed was 1.82, indicating spherical PM particles. The fractal dimension for the light duty engine was 1.65 and 1.7 for the traditional diesel and PCCI mode, respectively, indicating more spherical particles during PCCI mode. The TEM images were processed to obtain the compositions and microstructure of the PM particles. The PM particles observed contain mainly carbon as well as inorganic and metallic elements such as Ca, Mg, Zn, P and Si from engine oil, Al and Fe from engine wear, and S from fuel and lubricating oil. The compositions of PM particles were also found to vary significantly from particle to particle observed under identical operating conditions indicating the random burning of lubrication oil droplets and its effect on the formation of PM particles. The microstructure of primary particles from the heavy duty diesel engine exhibited two types of