Research On Key Thermo-Mechanical Characteristics Of Free Piston Engine Structure

As a novel internal combustion generator power system,the free piston engine for linear generator is attracting attention due to its advantages of simple structure,high power density,and short energy transfer path,etc,and it has become one of the preferred solutions for hybrid electric vehicles.However,the structural feature of the moving parts without mechanical constraints results in many problems in mechanism research and engineering application,and it is still at the exploratory research stage.This paper focuses on the structural thermo-mechanical characteristics of free piston engine.The piston movement and in-cylinder load characteristics were analysed.The energy conversion mechanism in one cycle was studied.The matching relationship between the structural parameters and operating parameters was resolved.On the basis of the above,the research on different thermo-mechanical characteristics of free piston engine in structure heat transfer,ring friction,piston stress and deformation was carried out.The main research objectives and conclusions are listed below:(1)Research on piston movement and in-cylinder load characteristics of FPEA numerical model of the FPE for the whole working process from starting to steady state was developed and validated using test data from the prototype.The piston movement and in-cylinder load characteristics were analysed.The results indicate that the shape of FPE piston speed loop is similar to a slanted rectangle with obvious piecewise linear feature.The peak value and the action time of FPE explosion pressure are less than that of crank engine.Compared with crank engine,the temperature of the gas in FPE cylinder is lower during the combustion process to the early stage of the expansion process,and is higher during the latter stage of the expansion process to the exchange process.(2)Research on the matching relations of FPE parametersThe form of energy conversion in different stages of FPE cycle was simplified and the mechanism of the effect of design parameters on FPE operating status was explored.The parameter matching expressions which meet the high operating efficiency and starting requirements were derived and validated by numerical methods.Operating frequency at optimal working point increases with increasing the cylinder diameter and decreases with increasing design stroke or moving mass.The requirement of the linear motor starting thrust was reduced with increasing stroke-bore ratio or moving mass.(3)Research on structure heat transfer characteristics of FPEA multi-discipline coupling simulation model was established based on the piston dynamics,in-cylinder thermodynamics and structure heat transfer,which was verified by the experimental results of the prototype.The effects of the piston movement and incylinder load on structure heat transfer were analysed,and the structure heat transfer characteristic of FPE was obtained.The structure temperature,the temperature gradient and the temperature fluctuation of the structure except the cylinder liner in FPE are lower than those in crank engine.Compared with crank engine,the heat flow density of the FPE combustion chamber wall is lower while the heat transfer area is larger at the same time in the expansion stage,and the proportion of the structure heat transfer loss between combustion air and cylinder liner is higher.The heat load is lower and the indicator thermal efficiency is higher in FPE.(4)Research on lubrication and friction characteristics of the piston ring for FPEFPE lubrication friction model involving the in-cylinder load,the piston motion and the structure temperature field was established.A new experimental method for deducing the friction work of the piston rings according to the motor thrust was proposed,by which the model was certificated.The lubrication and friction characteristics of piston ring for FPE were discussed,and the effect of different structure parameters of the ring on lubricating friction performance was obtained.Compared with crank engine ring,the friction loss of FPE ring is smaller while the peak friction power is larger.If the tubbish offset is negative,the peak of the friction force will be delayed to high speed moment,resulting in a higher peak of friction power.(5)Research on stress and deformation characteristics of the piston for FPEThe stress and deformation characteristics of FPE piston under thermo-mechanical loadings were calculated and analysed by finite element method.The influence rules of engine load and operating frequency on the stress and deformation of FPE piston were obtained.Compared with crank engine piston,the stress level and the skirt deformation of FPE piston are less.The maximum stress and the key points deformation of FPE piston are less affected by the engine load,however,they will increase nonlinearly with increasing operating frequency.