Modeling, Characterizing Of Solid-Body Contact At Meso-Scale And Its Application In Micro-Engine

With reducing the size of micro-device,a number of issues are being more and more notable,such as scale effect,surface/interface effect and coupling interaction.Given the issues above can bring about sealing,friction and wear etc.,which have become restraining factors for the development of micro-device.The practical interfaces of micro-device are a complex system,on the one hand,its size is macroscopic,on the other hand,some microscopic characteristics exist.Meanwhile,some behaviors of hierarchical,trans-scale,non-linear and multi-physics coupling can be exhibited,more importantly,greatly affect the design,response and service life of a system.However,the theoretical modeling and characterization method are still an open question at the meso-scale up to now,and lack of reliable measure technology.For these reasons,it is necessary to study the theoretical modeling,characterization method,evolution laws of the physical properties.Thus it can provide theoretical basis and analysis method for the design and development of the micro-device.The theoretical analysis of micro-engine with smooth surface can be performed firstly to reveal the important role and ubiquitous key problems for the interfaces,and further introduce the main work.Subsequently,a new mixed elastic-plastic model is developed to cover the shortage of contact status for individual sphere.For applying the contact status of individual sphere to rough surface,a new asperity-peak identification criterion is defined,and the asperity coalescence can be included by adding constraints.The evolution of the physical properties for rough surface profile will be studied.Finally,the present model can be applied to analyze the experiment results,and the feasibility of limitation size.The main contents done in this thesis are as follows:(1)Based on the equations of thermodynamic state and Navier-Stokes,and the theory of multi-body dynamics,the effects of various gap widths,engine speeds and contact parameters etc.on performances of micro-engine with smooth surface are investigated.The results show that the interfaces have significant effects on sealing,power etc,and the solid contact plays a more important role than fluid action.In view of the analytical results,it can be derived that the problems about the solid contact of rough surfaces need to do further research.(2)On the assumption that the contact behavior must be continuous,monotonic and smoothed,a new mixed elastic-plastic model is developed by using Hertz and Abbott-Firestone contact models,and validated through experimental results for copper and aluminum alloy reported in the literature,and compared with other contact models found in the literature.The present model shows good agreement with the measurements,and a big advantage especially for the greater displacements.Based on this,the calculation models of dynamic variables and energy dissipation are derived to predict the inter-relationships with applied load and material hardness etc.(3)In order to apply the contact status of individual asperity to rough surface,firstly regard the middle higher peak or lower valley as the true one in three continuous peaks or valleys based on the criteria of 3-point peak and 3-point valley,and then a new asperity-peak of “valley-peak-valley” mode is defined.This criterion can provide a baseline for the evolution of contact properties.The asperity interaction is achieved by adding the overlapping contact spots and solid energy,and merging according to the principle of equivalent volume.The results of asperity-peak properties,normal parameters of contact area and adhesion force,parameters of contact temperature and wear in the sliding process,and wettability,are validated through a set of different surface profiles,and compared with other criteria found in the literature.(4)The experiment of micro-engine is firstly proceedcd,and the variation law of performances of a spark-ignition with bore = 11 mm and a compression-ignition with bore = 9 mm can be obtained.The present model is validated through the test results of the power and contact temperature.Subsequently,the main performances of micro-engine are compared with traditional engines in various sizes.The lower possible miniaturization limits of micro-engine is investigated,and the results show that the micro-engine with cylinder bore = 2 mm,and even 1 mm proves to be feasible.The reasonable gap between piston with L shape and cylinder liner being 2 ~ 3 μm,intaking from the lateral of crankshaft,and the piston and crankshaft bush using graphite material are proposed after analyzing the performances for the cylinder bore = 2 mm.The theoretical model and characterization method proposed in this thesis provide a new research approach.The evolution laws at the meso-scale can be served as theoretical reference and design guide for miniaturization of micro internal combustion engine,and even development of other micro devices.