Study On Operation Characteristics And Working Mechanism Of Single-piston Freepiston Expander-linear Generator

As a new power machine,free-piston expander-linear generator（FPE-LG）has the advantages of simple structure,good balance,variable compression ratio and diversity of working medium selection,and has attracted attention of many scholars.FPE-LG has great application potential in the field of power and waste heat recovery,which can reduce the energy consumption and pollutant emission,and achieve the goal of peak carbon dioxide emissions and carbon neutrality.To find out the direction of reliable and stable operation,the starting characteristics are studied based on the single-piston FPE-LG test configuration.To clarify its working mechanism and energy consumption mechanism,the stability characteristics,force and energy consumption are studied based on the single-cycle.In addition,a numerical simulation model of single-piston FPE-LG is developed,and the main design parameters and main operation parameters are analyzed,which can provide the optimization direction for design and operation of single-piston FPE-LG.To find out the direction of reliable and stable operation,the operation characteristics of start-up process of FPE-LG are studied.The results show that during the start-up process,due to the force exerted on the piston by the gas in working chamber with piston rod（working chamber B）cannot overcome the inertia of piston assembly at the initial stage of intake stage,the over-expansion phenomenon will and only will appear in the working chamber without piston rod（working chamber A）.The over-expansion phenomenon under some working conditions will disappear during start-up process.The maximum in-cylinder pressure p _A is always less than intake pressure,and the maximum in-cylinder pressure p _B can exceed the intake pressure when the intake time is long enough.The in-cylinder pressure decreases in the later stage of the intake stage when the intake time is long enough or the intake pressure is large enough,On the whole,the in-cylinder temperature shows an upward trend and finally reaches stability.Locally,the in-cylinder temperature increases spirally,but with the progress of cycle,the temperature difference at the same state point of adjacent cycles decreases gradually.The start-up process can be divided into three stages:The movement and output reach stability in the first stage,the in-cylinder pressure reaches stability in the second stage,and the in-cylinder temperature reaches stability in the third stage.Under the condition of high intake pressure,the intake beginning positions has to keep away from both ends of cylinder and the intake time cannot be too long in case the cylinder collision.Under the condition of low intake pressure,FPE-LG cannot start if the intake beginning positions are too close to both ends of cylinder and the intake time is too short.In order to study the single-piston FPE-LG’s working mechanism,the stability characteristics in single-cycle are studied.The results show that the in-cylinder temperature changes periodically during stable operation.In the same cycle,due to the existence of piston rod,the contact area between the air in the working chamber on both sides of the piston and the piston is different,the maximum in-cylinder pressure,minimum in-cylinder pressure,the in-cylinder pressure and the maximum gas force are different,but the minimum gas force is equal.The velocity,bias displacement,one-way time,indicated work,peak voltage output,peak power output,electric energy output and work-electric conversion efficiency when the piston moves from top dead center（TDC）to bottom dead center（BDC）are different from those when the piston moves from BDC to TDC.With the increase of intake time or intake pressure,the energy entering working chamber increases,and in-cylinder pressure increases,which will increase the inertia of piston,so that the position of TDC moves towards the origin,the position of BDC moves away from the origin,and the bias displacement,stroke length,indicated work and velocity increases.Moreover,it also makes the peak velocity,the peak current output,peak voltage output,peak power output and the single-cycle electric energy output increases.Under the combined action of stroke length and velocity,the one-way time and period decrease firstly and then increase with the increase of intake time,and they decrease when intake pressure increases.The work-electric conversion efficiency increases firstly and then decreases with the increase of intake time at lower intake pressure,and it decreases with the increase of intake time at higher intake pressure,and increases with the increase of intake pressure.Under all working conditions have studied,the total work-electric conversion efficiency reaches the maximum value of 30.59%when intake beginning positions are 21-45mm,intake pressure p_in=4 bar and intake duration t_in=30 ms.Newton’s Second Law and the first law of thermodynamics are used to analyze the single-piston FPE-LG,and the change of four kinds of forces（air impetus,air resistance,electromagnetic resistance and frictional resistance）and work produced by the four kinds of forces are considered.During the operation of single-piston FPE-LG,in the same cycle,the values of electromagnetic resistance,frictional resistance and the work produced by the four kinds of forces when piston moves from TDC to BDC are different from those when the piston moves from BDC to TDC.As the power of air entering working chamber increases with the increase of intake time or intake pressure,the power capacity of air in working chamber can be improved.However,due to the limitation of intake and exhaust tube diameter and stroke length,the air resistance increases as well.The increase of air impetus will increase the velocity,which makes the electromagnetic resistance and frictional resistance increase.The increase of power capacity increases the inertia and stroke length,which increases the work done by the four forces.But the one-way nominal work-electric conversion loss coefficient and the total nominal work-electric conversion loss coefficient decrease firstly and then increase.The minimum proportion of energy consumption caused by air resistance account for more than 50%,the proportion of energy consumption caused by electromagnetic resistance account for less than 20%,and the proportion of energy consumption caused by frictional resistance account for more than 20%.Reducing the air resistance energy consumption will increase the electromagnetic resistance energy consumption,frictional resistance energy consumption and the electric energy output.Therefore,the total nominal work-electric conversion loss can be effectively reduced by increasing intake time or intake pressure in a certain range.Under all working conditions have studied,the total nominal work-electric conversion loss coefficient reaches the minimum value of 0.8613 when intake beginning positions are 21-49mm,intake pressure p_in=4 bar and intake duration t_in=60 ms.Based on the basic parameters of single-piston FPE-LG,a numerical simulation model is developed.The intake process and exhaust process are considered in the model.The influence of main design parameters and main operating parameters on performance parameters and the mechanism are investigated,and the sensitivity is analyzed.The research shows that simulation results are in good agreement with experimental results,and have high reliability,which can be used to performance research under different working conditions.The force condition of piston is mainly affected by the design parameters,such as piston assembly mass,back EMF constant,frictional resistance coefficient,piston rod diameter and cylinder diameter.The air intake mass of the system is affected by the operating parameters,such as intake beginning position,intake time and intake pressure.The operation characteristics are affected by the above parameters,and the external load also affects the operation characteristics by affecting the electromagnetic resistance coefficient.The parameter sensitivity analysis shows that among all design parameters,the power output and operating frequency are the most sensitive to the cylinder diameter.Among all operating parameters,the power output and operating frequency are the most sensitive to the intake pressure and the intake beginning positions.To increase the power output,it can be considered to increase the values of parameters such as back EMF constant,cylinder diameter,intake and exhaust pipe diameter,intake pressure and intake time,reduce the value of piston rod diameter,and take values of parameters such as piston assembly mass,friction resistance coefficient,external load and intake beginning position within a certain range.To increase the operation frequency,it can be considered to reduce the values of parameters such as piston assembly mass,back EMF constant and piston rod diameter,increase the values of parameters such as cylinder diameter,intake beginning position A,intake pressure and external load,and take values of parameters such as friction resistance coefficient,intake and exhaust pipe diameter,intake beginning position B and intake time within a certain range.