Theoretical And Experimental Studies On A Miniature Free Pistonlinear Engine

The past decade has witnessed a rising demand for Micro-Electro-Mechanical Systems(MEMS)with the rapid development of science and technology.And these portable devices heavily relay on batteries.Unfortunately,the batteries have disadvantages like low energy density,large mass or volume,short power supply time,and these disadvantages greatly limits the developments of the MEMS.Therefore,it is necessary to develop the long lifetime and high energy density miniature energy generation system to replace the lithium batteries.As we all know,the specific energy of hydrocarbon fuel is very high,typically 50 MJ/kg.Besides,the hydrocarbon fuels also have advantages such as fast fuel supplement,environment-friendly combustion products.Therefore,the miniature power generation system based on the fuel combustion has become the most promising alternative to lithium batteries.In this paper,a miniature heat engine based on gaseous hydrocarbon fuel(such as methane)has been developed.Among various miniature heat engine concepts,a dual-piston miniature free piston liner engine(FPLE)in this paper is selected.The FPLE has a moderate running frequency and can be directly coupled with the liner generator,thus can utilize the engine volume more effectively and contribute to the promotion of generating efficiency.In addition,two power strokes can be obtained in one operating cycle due to the dual piston layout,resulting in higher power density.In this work,the miniature free piston linear engines have been designed and manufactured,moreover,the theoretical and experimental studies of the FPLE system were carried out.In chapter 2,the dynamic and thermodynamic analyses are investigated by analyzing the operating characteristics of the free piston engine.A control volume is assumed around the entire miniature FPLE,and the first law analysis of energy balance on the engine has been conducted.Results taken from the experiment is used to perform verification of scavenging model.The principles of strength and modal of the mounting are present,and the relationship between the mounting height and deformation also is analyzed.In chapter 3,the modeling and simulation of the FPLE were presented.It consists of thermodynamic model and dynamic model,and the effects of heat transfer,friction and electromagnetism on the running characteristics of the engine are considered.Then the basic features of FPLE were learned from the simulation.The stress and modal analysis of free piston engine support system was then carried out,and the size optimization of the mounting was completed.Based on the electromagnetic theory,the basic parameters of linear generator were determined and the linear generator prototype were then made.Test results showed that the linear motor could meet the design requirements.In order to improve the engine durability,a splash lubrication strategy for the miniature free piston engine is proposed.Experimental results showed that the lubrication strategy is feasible for miniature free piston linear engine.In addition,the connected rod and corresponding anti-rotation device,cylinder sleeve,spark system and fuel supply system were also developed.In chapter 4,the experimental platform for FPLE with glow plug ignition was set up.The first-generation FPLE prototype works on two strokes with glow plug ignition.The experimental results showed that the FPLE could be started successfully by hand and operate continually.However,the prototype showed poor performance duo to the ignition signal delay.In the following study,the problems exposed in the first-generation prototype were solved and engine structures optimization have been completed.Taking the improved prototype as the research object,the influences of glow plug heating power,piston assembly mass,mixture flow rate and equivalent ratio on the FPLE performances were comprehensively studied.The sensitivity analyses of indicated power and indicated thermal efficiency to the variation of external parameters were completed.The characteristic of compressed air energy storage and analysis of disturbance source from external load and mixture flow rate were also performed.Furthermore,based on the principle of equal scale,the miniaturization design of the first-generation FPLE was carried out.And the second-generation FPLE was developed.The engine fueled with butane has a displacement volume of 1.6 cm~3 and works on two-stroke cycle,which is the smallest one ever reported in the literatures.The thermal balance analysis of the second-generation was then analyzed and the different loss items were quantitatively evaluated,which provided meaningful references for the FPLE improvement and optimization.Finally,based on the operating characteristics of two generations FPLE prototypes,the sensitivity analyses of operating frequency,piston speed,mean compression ratio,indicated power and indicated thermal efficiency to the scale variation were present.In chapter 5,based on the first-generation FPLE,effects of hydrogen addition on the engine performance and emission characteristics were investigated both experimentally and numerically.Firstly,the influences of hydrogen volume fractions(0%-5%)on the FPLE performance(operating frequency,indicated power,indicated thermal efficiency,heat release rate,pollutions emission,etc.)were obtained.The experimental results indicated that the hydrogen addition can significantly enhance the engine performance and induce soot formation.secondly,the CONVERGE soft was used to build the 3-D cylinder simulation model for the combustion kinetic analysis.Then the model verification was completed using experimental data.With the numerical results,the internal mechanism of hydrogen addition on the engine performance and emissions were well revealed.