| Biofuel is renewable, clean energy and known as a potential substitute engine fuel for conventional fossil fuels. If fuel soot enter into lubricating oil system of engine, soot particles may lead to the increase of viscosity of lubricating oil, more sludge, reduction of wear resistance, increase of oil filter pressure, even drop offilter clogging.Consequently, it affects the lubricity and usability of engine. The surface properties of soot particles are associated with the types and compositions of fuels. The present dissertation is mainly focused on the surface performance of biofuel soot particles (BS)and its effects on the dispersibility, adsorbability and tribological performanceadsoption of engine lubricating oil in order to enhance the application of biofuel as engine fuel.A set of experiment equipment was designed and manufactured to trap soot particles, in which BS and diesel soot (DS) particles were collected by burning the biofuel and 0# diesel fuel respectively. Their surface morpholoies and physical and chemical properties were characterized contrastively. A four-ball tribometer was used to investigate the effects of BS on the tribological performance of liquid paraffin (LP,simulant of base stock for lubricating oil). The surface morphologies, compositions,structures, and the mechanisms of friction and wear of BS and DS were investigated by way of analytical techniques. The groups on the BS and DS surfaces were compared by means of Fourier Transform Infrared Spectrometer (FTIR). The structural analyses of BS and DS were conducted using a Rigaku X-ray diffraction (XRD). The particle size distribution and average particle diameter of BS and DS dispersed in n-heptane were analysised by Zeta potentiostat. The degrees of graphitization of BS and DS were conducted by Raman Spectroscopy. The surface morphologies and element contents on the worn ball surfaces were investigated by scanning electron microscopy with energy-dispersive spectroscopy (SEM/EDS). The micro-structures of BS and DS particles were examined using high resolution transmission electron microscopy(HRTEM). The chemical states and intensities of the elements on the worn surfaces were investigated with X-ray photoelectron spectroscopy (XPS). Results showed that BS and DS were of chain-like or cluster-like agglomeration which was composed by spherical or nearly spherical nano-particles with average diameter of 38-40 nm.Both BS and DS were mainly composed of amorphous carbon with superfined graphite turbosratic structure. The degree of graphitization of BS was higher than that of DS. The main elements of BS and DS were oxygen and carbon.There were some polar functional groups with oxygen on the BS and DS surfaces. The oxygen content on the BS surface was higher than that of DS. The average particle size and the size range of BS were smaller than those of DS in the solvent of n-heptane. The extreme pressure value (PB) of LP initially increased and then reduced with the increase of BS content in LP. The average wear scar diameters (AWSD) increased linearly and the average friction coefficient decreased slightly and then increased with the increase of BS content in LP. Wear mechanism analysis showed that the functional groups containing oxygen in BS might easily adsorb on the rubbing surface and influenced the effect of boundary lubrication film. Meanwhile, the metal oxide on the rubbing surface might become abrasive particles easily and increased wear due to the BS roles of abrasive and corrosion wear.A four-ball tribometer was selected to investigate the active control effect of nano-sized rare earth compounds on the tribological performance of BS particles. It was studied the effect of surface modified nano-sized lanthanum fluoride particles(nano-LaF3) on the suface adsorption behavior of BS on its tribological property. The structural analysis of surface-modified nano-LaF3 was conducted using XRD. The surface morphology and element contents on the worn ball surface were investigated by SEM/EDS. It was aslo studied that the chemical states and intensities of the elements on the worn surfaces and BS before and after rubbing process, and the active element contents on the surface of cylinder line after static adsorption experiment by means of XPS to interpret the possible influence mechanism of nano-LaF3 on the tribological behavior of BS-contaminated LP. The results showed that the AWSD increased with BS,regardless of with or without nano-LaF3. The optimum content of nano-LaF3 was 0.6 wt% in the LP with 2.0 wt% BS. The nano-LaF3 significantly contributed to the enhancement of the anti-wear and antifriction properties of BS-contaminated LP,even also to PB. The possible mechanisms were attributed to the formation of a boundary lubrication film including iron oxides,lanthanide and carbon.On the other hand,the preferential adsorption of nano-LaF3 on the surface of BS particles weakened the corrosion wear on the worn surface.The surface properties of BS and DS were investigated contrastively by means of XPS, FTIR, full-automatic micropore physisorption and chemisorption analyzer, Zeta potentiostat and optical contact angle/interface tension meter in order to study the effects of surface properties of soot particles on the viscosities of LP/BS and LP/DS respectively. Results showed that the relative viscosities of LP/BS and LP/DS increased with the soot content by exponential function at 40?. The relative viscosity of LP/DS was higher than that of LP/BS in case of same soot content. Higher soot concentration might lead to the more obvious behavior of shear thinning. The viscosity of LP/DS was more severely affected by shear rotation speed. The surface analysis results showed that there were some O-containing functional grops on the surfaces of BS and DS. The surface oxygen content on the DS surface was less than that of BS. The specific surface area and the surface energy of DS were higher than those of BS. The lipophilicity of DS was less than that of BS. Thus, DS was apt to agglomerate into larger agglomeration particles in LP and DS affected LP viscosity more severely than that of BS.It was studied the influence of ashless dispersant (polyisobutylene succinimide,namely by T154) on the dispersities of BS and DS in LP by means of the viscosity method, particle size distribution and spot experiments respectively. The mechanisms of BS dispersed by T154 were conducted by means of FTIR and XPS. Results showed that T154 can disperse BS and DS into LP effectively. However, the effect of T154 on the BS/LP was better than that of DS/LP. T154 can also be acted as a dispersant to disperse BS into lubricating oil. Since there are more acid groups on the BS surface, such as carboxyl, phenolic hydroxyl and so on, it can take place the acid-base action with succinimide group of dispersant in T154, or form hydrogen bonds with N-H unit in T154. Meanwhile, the polyisobutylene group in T154 formed steric hindrance in LP,which could prevent BS from aggregation and stabilized the BS particles dispersed in the lubricating oil.It was investigated that the influences of three kinds of calcium salt detergents on the dispersity of BS in LP by four simulation methods (viscosity, spot, static sedimentation and particle size distribution). The calcium salt detergents included the synthesis calcium sulfonate with higher total base number (T106), alkyl calcium salicylate (T109) and sulfuration calcium alkylphenol with higher total base number(T115B). It was studied that the effect of calcium salt detergent on the disperse mechanism of BS by means of XPS and FTIR. Results showed that the BS/detergent/LP systems with 4.8 wt% BS presented the obvious behaviour of shear thinning, which were similar with that of non-Newtonian fluid. At the same time, the dispersion system of BS/T109/LP showed the minimal dynamic viscosity, the maximum sludge dispersion threshold, the largest drop ratio of the height of supernatant liquid, the minimum size range and average particle size of aggregated particles in n-heptane. Therefore, T109 was the excellent additive to disperse BS in LP. The mechanism analysis showed that the O-containing polar groups (such as carboxyl or hydroxyl) on the surface of BS were liable to adsorb detergent through hydrogen bond or acid-base function.At the same time, the good lipophilicity of non-polar chain alkane groups of detergents formed steric hindrance in LP, which impeded the BS particles' aggregation. Consequently, the detergent can also disperse the BS particles in the lubricating oil well... |
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