Research On Operation Strategies And Multi-module Application Of Free-Piston Engine Linear Generator

Free-piston engine linear generator(FPLG)is a new type of energy conversion devise based on the direct coupling of free-piston engine and linear generator.FPLG gains wide attention because of its simple structure,high power density,high efficient energy conversion and simultaneous operation of many modules.Which is in line with the advocating of saving energy and protecting the environment.However,the study of current FPLG is still under exploration.Therefore,there are many problems in the theoretical studies and technological innovation.This paper at first studies the precise models of sub-systems of the FPLG,and further explores the influences of the parameters of different subsystems on the mover dynamic characteristics and the performance of FPLG.Then,we designed the controlling strategies for different working processes of the FPLG based on the FPLG operation principle.This paper also compared the mover dynamic characteristics and FPLG performance based on experiment results from the improved and optimization FPLG physical prototype.The heat-release rules during the generating process and the combustion circulation were also studied.Finally,this thesis studied the dynamic features of the actual vehicle with multi-modules of FPLG.The following are the main study conclusions.(1)Based on the linear generator operation theory,precise and accurate linear engine model was established by equaling linear engine model with rotating generator model.Precise friction was calculated based on the viscous shear stress and frictional shear force in the lubrication region.Then,the simulation results and experimenting results of in-cylinder gas pressure and single phase voltage were compared to verify the precision of the model.Furthermore,the influences of the parameters of each subsystem of FPLG on the mover dynamic characteristics and FPLG performance were analyzed.(2)According to the working process of FPLG,the working process is divided into starting process,switching process and generating process.Because the motor of FPLG has four-quadrant operation features,the vibration starting controlling strategy was designed based on resonance principle of mechanical system.Compared with simulation results,it is suggested that the controlling strategy can realize successful starting.Meanwhile,simulation results suggest that as the thrust increases,the in-cylinder gas pressure and the compression ratio showed non-linear increase,and the circulation times needed to reach ignition condition declined.(3)This paper also studied the switching strategies,switching time and response time during the FPLG switching process.First,the “immediately switching” strategy and “gradually switching” strategy were compared.The results suggest that the mover displacement and velocity was gradually slower when applying “gradually switching” strategy compared with “immediately switching” strategy.In response to the switching timing,this paper compared the changes of motor thrust at the TDC position timing,middle stroke timing and random position timing during the FPLG switching process.The results suggest that changing the motor thrust at the TDC position timing had the minimum effects on the operation of FPLG,and changing the motor thrust at the middle stroke timing had the maximum effects on the operation of FPLG.In terms of the switching response time,study suggests that the switching response time was far less than the operation cycle of the FPLG,and it can be neglected.It is also found that no matter which strategy or switching position timing,the mover TDC and the velocity peak after switching were lower than that before switching.Moreover,the mover TDC and velocity peak after switching process were the same.(4)While the FPLG during the generating process,the motor subsystem controlling strategy was adjusted to maintaining stable strategy.According to the motor vector controlling strategy,displacement ring-speed ring-electric circulation ring and SVPWM calculation controlling strategy was applied to adjust the motor force,based on which the FPLG fixed the mover trajectory operation controlling strategy was designed.Results suggest that the fixed trajectory operation controlling strategy exerted favorable effects on ensuring the stability of FPLG,which further verified the feasibility of fixed trajectory operation controlling strategy.(5)In this study,the physical prototype was improved and optimized based on theoretical analysis.The mover dynamic characteristics and FPLG performance at different working processes were analyzed.Experimental results suggest that the parameters of the TDC,velocity peak and operation parameters during the generating process were lower than those during the dragging combustion process,but larger than those during the vibrating starting process.In the same time,all the parameters of the subsystems of engine during the generating process were smaller than the parameters during the dragging combustion process.Moreover,the post-combustion of the FPLG was obvious.(6)By using mixed driving form with storage battery and multi-modules of FPLG,and based on the structure parameters and the performance parameters of the vehicle,we calculated the performance parameters of corresponding parts.In the energy management strategy,we used fuzzy logic controlling strategy to set up the energy distribution ratio of the multi-module FPLG output power and storage battery output power.Simulation results suggest that the energy from the multi-module FPLG can satisfy the demands of power for vehicle during the driving process.Furthermore,the storage battery played a complementary role during the driving process under the premise that the storage battery was charged and then used to drive the vehicle.