Abstract
In this work, an investigation of soot-in-oil samples drawn from the oil sump of a gasoline direct injection (GDI) engine was carried out. Soot particulate was characterised in terms of size, distribution and shape of the agglomerates, and internal structure of the primary particles. The test engine was a 1.6l modern light-duty EURO IV engine operated at speed between 1600 and 3700rev/min, and torque between 30 and 120Nm. After a double oil-flushing procedure the engine was operated for 30h. Oil samples were drawn from the sump and prepared for Transmission Electron Microscopy (TEM) and High resolution TEM analysis (HRTEM) by a combination of solvent extraction, centrifugation and diethyl ether bathing. Soot agglomerates were measured in terms of their skeleton length and width, and fractal dimension. The mean skeleton length and width were 153nm and 59nm respectively. The fractal dimension was calculated using an iterative method and the mean value was found to be 1.44. The primary particles were found to be spherical in shape with some irregularities and presented an average diameter of 36nm with a mode of 32nm and standard deviation of 13nm. The majority of particles showed an inner core and outer shell similar to diesel soot, although an amorphous layer was also clearly visible.
Highlights
Gasoline Direct Injection (GDI) engines are considered an important source of carbonaceous nanoparticles; they produce higher levels of soot as the process of fuel vaporisation and gas-phase mixing remains essentially incomplete [1], even when early fuel injections are used to enable a homogeneous combustion mode [2,3]
Similar particles were found in diesel soot-in-oil samples in [20] and were considered to be oil additives
Lsk and Wsk were inferred from 2-D Transmission Electron Microscopy (TEM) images using an open architecture image processing software, ImageJ, whilst a MATLAB algorithm was created for calculating the fractal dimension
Summary
Gasoline Direct Injection (GDI) engines are considered an important source of carbonaceous nanoparticles; they produce higher levels of soot as the process of fuel vaporisation and gas-phase mixing remains essentially incomplete [1], even when early fuel injections are used to enable a homogeneous combustion mode [2,3]. Nanoparticle tracking analysis has recently been employed for the measurement of soot agglomerates size distribution and number density from automotive engines oils [16]. Barone et al [30] used TEM to investigate the diameter of aggregate primary particles from GDI exhaust gas soot. They studied particles morphology as a function of injection strategy. Soot-in-oil from GDI engine has not been investigated widely; to the authors' best knowledge, its agglomerate size distribution and shape have not been reported in the literature. As this technique requires the beam to impinge on the sample for a greater length of time, this technique benefits from the sample cleaning process developed for HRTEM
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