Pacino, Andrea
(2024)
Morphological and structural characterization of soot-in-oil samples and their impact on marginally lubricated engine components.
PhD thesis, University of Nottingham.
Abstract
The major drivers in the development of the latest generation of engines are environmental. For diesel engines, mitigating the effects of soot contamination remains a significant factor in meeting these challenges. There is general consensus of soot impacting oil performance. Considerable efforts have been made towards a greater understanding of soot-lubricant interaction and its effects on engine performance. However, with evolution of engine designs resulting in changes to soot composition/properties, the mechanisms of soot-lubricant interaction in the internal combustion engine continue to evolve. A variety of mechanisms have been proposed to explain soot-induced wear in engine components. The first part of this work aims to critically review and discuss the current understanding of soot-induced mechanisms in heavy-duty diesel engines, as reported in the literature. Emphasis will be given to the aspects of wear, friction, and viscosity, critically highlighting the main pathways for future research. Different hypotheses on wear and the potential mechanisms behind the soot-lubricant interaction are also discussed, showing potential issues related to soot contamination as well as the strong relationship with oil formulation. Multiple soot properties are responsible for wear and their impact seems to depend on the boundary lubrication conditions achieved during the test. Therefore, a systematic soot characterization from different engine test conditions is required for a comprehensive assessment of soot impact on engine components.
Thus, the morphology, nanostructure, and composition of soot extracted from the oil sump of different heavy-duty engines operated under dynamometer and field conditions were investigated. Soot characteristics were then compared to three carbon black materials. Soot was extracted from used oils for Transmission Electron Microscopy (TEM) analysis. Energy Dispersive X-Ray (EDX) and X-ray photoelectron spectroscopy (XPS) analysis were also performed to assess soot composition. Two soot classes, I and II, can be identified based on how they appear under the TEM. Carbon blacks and class I particles have graphitic structures, while class II samples have a more sludge-like appearance. Similar aggregate sizes were observed among the samples. Primary particle size shows a unimodal distribution in all samples, with a median particle diameter from 18 nm to 22 nm. Differences in the length and tortuosity of the graphitic fringes between the samples were observed. The findings suggest a greater degree of interaction between class II samples and the lubricating oil, and consequently, a different wear behaviour is expected depending on the specific mechanism involved.
Additional research was conducted on artificially aged oil samples loaded with 1% carbon black (Monarch and Vulcan) to investigate the observed sludge-like formations in used oils. Results show that when 1 wt.% Monarch was present in the oil during the ageing process, a thick amorphous layer was observed on CB particles in comparison to the test in which the same amount of CB was added to the aged oil.
To further investigate the impact of soot on real engine components, the most similar CB sample was used as soot replica during chassis dynamometer testing. A chain wear test rig is used to motor a 1.3 L diesel engine following the speed profile of a Worldwide Harmonized Light Vehicle Test Cycle (WLTC). The lubricant oil was loaded with 3% carbon black of known morphology. The chain length is measured at regular intervals of 20 WLTC cycles (i.e. 10 hours) and the wear is expressed as a percentage of total elongation. Oil samples were collected and analysed with the same frequency as the chain measurements. Carbon black morphology and nanostructure were investigated using Dynamic Light Scattering (DLS) and TEM. DLS data revealed that carbon black particle size did not change substantially in the first 10 hours, however, during the remaining test cycles a reduction in agglomerates size over time was observed. The wear results show that adding carbon black to the lubricating oil promotes chain elongation by up to 0.10%. Significant chain elongation occurred within the first 10 hours (+0.06%), with further increase in elongation occurring in the remaining 40 hours (+0.04%) but under a reduced wear rate. The overall results suggest that dynamically changing carbon black size distributions and nanostructure could be linked over time.
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