Fundamental Studies Of Supercritical CO₂ As A Cleaning Process For Diesel Engine Remanufacturing

Remanufacturing is the rebuilding of a used product to at least its original performance with a warranty that is equivalent or better than that of the newly manufactured product.It is a vital means in improving manufacturing sustainability,through saving raw materials,reducing carbon emissions and conserving product commercial values.As the one of the demanding steps in remanufacturing process,cleaning is also one polluting stage because of the use of environmental detrimental cleaning agents and the large amount of energy consumptions.The thesis summarises commonly used cleaning processes,based on which the thesis aims at proposing an environmentally friendly process for diesel engine remanufacturing cleaning operation-supercritical carbon dioxide(SC-CO2)cleaning-to decrease the environmental impact of cleaning operations in remanufacturing industry.The physiochemical properties of contaminants are analysed by use of Fourier Transform Infrared Spectroscopy(FTIR).Electron Probe Micro-Analyser(EPMA)and element analysis,which validates the effectiveness of supercritical fluid cleaning.The analysis of contaminants from decomrmissioned engine parts demonstrates that massive organic contaminants remain on the engine part surfaces after long time services,e.g.lubricating oils and greases,carbon depositions and organic paint layers.Therefore,the investigation is carried out on these three types of contaminants using supercritical experiment apparatus.The work in this thesis initially studies the removal rate of SC-CO2 cleaning under a multi-parameter condition and reveals the competitive mechanism of temperature and pressure on the removal rate.Accordingly,a factorial study based on Design of Experiment(DoE)is introduced to investigate the crucial parameters in the cleaning process,with the employment of Pareto Principle.This study has succeeded in proposing a model to predict the cleaning effect under different operating conditions for engine contaminant cleaning,for the first time.Following on from the previous study,a model is established to describe the process in SC-CO2 cleaning and different mechanisms are analysed for different types of contaminants on the engine parts.The comparison between SC-CO2 cleaning and conventional pyrolysis cleaning demonstrates that the two cleaning processes have similar mechanism,i.e.primarily decreasing the adhesive forces between contamination layers and substrate materials by material removal process.And the residuals after SC-CO2 or pyrolysis are removed by subsequent mechanical agitations.Results have proved that pyrolysis cleaning could cause damages to the substrate materials while the SC-CO2 cleaning is able to maximumly preserve the original properties of engine parts.Ultimately,the comparative life cycle assessment is conducted between two cleaning processes:SC-CO2 cleaning combined with liquid blasting(SCB)and pyrolysis with shot blasting(PYB)r.which is the cleaning process currently used by automotive remanufacturers.Results have confirmed that the SCB process possessed better environmental performances in attaining a same cleanliness level,in comparison with the PYB process.Assessment result also give recommendations to furtherly reduce the environmental impacts of either cleaning process,i.e.to use green energy.It is concluded that the SC-CO2 cleaning could provide a"cleaner production”method for remanufacturing.