| Due to the urgent traditional fuels demand and the increasing environmental concerns,there has been more focus on the renewable fuels for the replacement of petroleum.Biodiesel is one of renewable engine fuels and regarded as an excellent substitute for petrodiesel.Biodiesel is comprised of a variety of long-chain fatty acid alkyl esters including saturated and unsaturated compounds and typically made from raw materials such as vegetable oils,animal fats or waste cooking oils.Engines powered by biodiesel can benefit from reduced particulate matter and green house effect.However,the generation of soot is a by-product of the combustion process that cannot be eliminated easily.Most of the biodiesel soot?BDS?generated during the combustion process is exhausted,but some can contaminate the lubricating oil system of engine and lead to increase the viscosity of lubricating oil and engine wear.The energy saving and service life of engines are significantly affected by the influence of BDS on the lubrication performance of engine lubricating oil.Therefore,it should be further explored how to control the effect of BDS on the tribological properties of engine lubricating oil.In this thesis,basic physical-chemical and tribological properties of BDS were investigated on the basis of green synthesis of biodiesel.Dispersion of BDS,its influence on tribological behavior of lubricating oil and relevant mechanism were explored.The effect of multi-solid particles on the tribological behavior of BDS was also studied.Furthermore,the transformation of BDS into onion-like particles and the lubrication mechanism of onion-like BDS were investigated as well.This study is helpful for the active controlling for the tribological property of BDS.Firstly,the green biomass ash catalyst for the preparation of biodiesel was produced by cedar ash.The catalyst was characterized by Inductively Coupled Plasma Mass Spectrometer,Simultaneous Thermal Analyzer,X-ray Diffracmeter,Field Emission Scanning Electron Microscopy and Hammett Indicator.The effects of catalyst preparation and transesterification reaction conditions on the catalytic performance were investigated.The results showed that the optimal conditions of first calcination temperature,first calcination time,second calcination temperature and second calcination time were 800°C,2 h,500°C and 2 h,respectively.The active species of cedar ash catalyst under these conditions was CaO and the base strength of cedar ash catalyst was between 9.8 and 15.0.The cedar ash catalyst exhibited a high catalytic performance for the synthesis of biodiesel by transesterification reaction.The biodiesel yield reached 91.52%under the optimal conditions with catalyst mass fraction of 7 wt%,catalytic time of 5 h,methanol/oil molar ratio of 14:1 and catalytic temperature at 65°C.After four-time regeneration of the cedar ash catalyst,the biodiesel yield still reached82.03%.Secondly,BDS and No.0 diesel soot?DS?were obtained by the combustion of biodiesel and No.0 diesel at normal temperature and atmospheric pressure.The physical-chemical of BDS and DS were analyzed.It was emphatically investigated the effect of BDS on the tribological behavior of liquid paraffin?LP?,and the tribological mechanism was also detailed.The results indicated that chain-like aggregation of BDS and DS were consisted of a large amount of near-spherical primary particles and the average primary particle diameter of BDS?35 nm?was smaller than the average primary particle diameter of DS?39 nm?.Both BDS and DS were composed of graphitic layers and amorphous carbon.The degree of graphitization disorder of BDS?ID/IG=2.937?was lower than that of DS?ID/IG=3.162?.BDS contained more carbon content and less oxygen,hydrogen,nitrogen and sulfur content than DS.The groups?C-C?C-O-C and C-OH?were presented on the surfaces of BDS and DS.Moreover,only BDS did not contain C=O group.With increasing BDS or DS content,the dynamic viscosity of LP increased.The relative viscosity increased by exponential function with increasing the mass fraction of BDS or DS at 20°C.The relative viscosity of LP contaminated by BDS was higher than that of DS when the mass fraction of soot was higher than 1 wt%.In terms of the aggregation mechanism,compared with DS,BDS possessed higher surface energy and less lipophilicity which was the main reason for BDS to be agglomerated into larger aggregation particles in LP easily.BDS could enhance the PB value of LP.BDS decreased the wear resistance of LP.The friction-reduction property of LP improved when the content of BDS was below 1 wt%.However,when the content of BDS exceeded 1 wt%,BDS decreased the friction-reduction ability.The tribological mechanisms could be summarized as follows:at low BDS content,BDS acts as a friction modifier to reduce the friction coefficient.However,at high BDS content,BDS aggregation leads to increase of abrasive wear role and influence the lubricating oil film,which could reduce the abilities of friction reduction and wear resistance of LP.Thirdly,the relationship between additives?polyisobutylene succinimide dispersant?PSD?,alkyl calcium salicylate?CSD-1?,high alkaline calcium sulfonate?CSD-2?and sulfuration calcium alkylphenol?CSD-3??and the dispersion of BDS was studied.The influence of BDS dispersion on the tribological behavior of LP containing BDS was investigated.Moreover,the dispersion and tribological mechanism of BDS were also studied.It was found that the BDS that adsorbed?PSD+CSD-2?had the highest content of–OH group,the least average particle size in LP compared with single additive.Combination addition of PSD and CSD-2 caused a synergistic effect.The wear volume of LP with 9 wt%?PSD+CSD-2?and 5 wt%BDS decreased from 7.73×105?m3 to5.42×105?m3,it meant the wear volume of LP decreased by 29.9%.The mechanisms analysis showed that?PSD+CSD-2?adsorbed on the surface of BDS through the hydrogen bonds and impeded the particle aggregation of BDS.It also dispersed and solubilized the BDS particles in LP.Hence,?PSD+CSD-2?could improve the antiwear performance of LP with BDS.Fourthly,the tribological properties and wear mechanisms of rGO@Fe3O4 as lubricant additive in PAO6 base oil containing BDS were studied in which rGO was reduced graphene oxide.The results indicated that rGO@Fe3O4 could improve the antiwear property of PAO6 base oil containing BDS.The wear volume of PAO6 base oil including 0.5 wt%rGO@Fe3O4 and 5 wt%BDS decreased from 1.92×106?m3to1.25×106?m3?decline extent of 34.9%?under rubbing 10N load and 10 mm/s sliding speed for 30 min.The tribological mechanisms can be attributed to rGO@Fe3O4 acting as spacing between the friction surfaces to prevent the direct contact of asperities between the friction pairs or between BDS with the friction pairs.In addition,the Fe3O4nanoparticles from rGO@Fe3O4 can fill the rubbing zone to serve as role of ball bearing.Finally,the onion-like carbon nanoparticles derived from BDS were prepared through acidified treatment process.The tribological properties and mechanisms of nitric-acid-treated BDS?NA-BDS?in water and NA-BDS modified with oleylamine?NA-BDS-OLA?in LP were investigated respectively.The results showed both NA-BDS and NA-BDS-OLA were onion-like carbon nanoparticles with an average size of 35 nm.NA-BDS and NA-BDS-OLA had a lesser degree of graphitization disorder than BDS.NA-BDS and NA-BDS-OLA could effectively strengthen the antiwear and friction reduction properties of H2O and LP,respectively.The tribo-mechanisms of NA-BDS and NA-BDS-OLA could be attributed to the spacing,ball bearing and exfoliation effects.Both NA-BDS and NA-BDS-OLA could avoid the direct contact between friction pairs asperties and acted as ball bearings between the rubbing surfaces at a low load.Moreover,the exfoliation effect led the formation of a nano-graphitic layer from both soot partticles,which decreased the friction and wear because of the adhere role of nano-graphite on the rubbing surfaces and facilitated easy shearing at high load. |
PDF Full Text Request