Study On Resonant Start-up And First Cycle Characteristics Of The Free-piston Linear Range Extender

Free-piston Linear Range Extender(FPLRE),as a new type of green power device,has many performance advantages such as compact structure,variable compression ratio,and short energy transfer path,and has attracted more and more attention from research institutions under the background of "double carbon" that vigorously promotes energy conservation and emission reduction,and has become the preferred solution for new energy power devices.The special construction of the "crankless" is the reason for the many advantages of the FPLRE,and the free movement without mechanical constraints also makes the start-up process a new challenge.As the first combustion cycle of the starting process,the first cycle is directly affected by the starting working conditions,and the defects such as low temperature and pressure,and difficult mixture formation make incomplete combustion or even fire occur from time to time,thereby worsening the economic performance and emission performance of the starting process.In this context,this thesis takes the two-cylinder opposed FPLRE as the research object to jointly study its resonant start-up and first cycle stage.The joint simulation is carried out on MATLAB/Simulink and AVL＿FIRE software platform to construct the FPLRE machine model,and the special piston motion law is obtained,and the FPLRE operation process(compression,combustion,expansion,etc.)is accurately simulated by its multi-field coupled CFD model.Based on this,the influencing factors of the running characteristics and starting performance of the start-up process are discussed,the energy conversion mechanism of this process is analyzed,the energy dissipation in the cycle is obtained,and the optimal value of the starting thrust is analyzed according to the nonlinear dynamic characteristics of the start-up process.The core content and specific conclusions of the study are as follows:(1)Aiming at the problem of difficulty in starting FPLRE,a starting strategy of reciprocating oscillation accumulated energy is adopted,and the operating characteristics of FPLRE in the resonance start-up stage are studied.In addition,the starting performance was studied based on changes in starting thrust,frictional load,and piston sealing performance.The results show that the FPLRE start-up process is a self-excitation vibration process,during which the piston movement fluctuates greatly to gradually stabilize.During the starting process,if the free piston moves in the state of excellent sealing performance,small friction load and large starting thrust,the effective stroke of FPLRE is extended and the piston moves rapidly,which can achieve the ignition condition in a short enough time to achieve the successful start of the device.(2)The resonant start-up process of FPLRE is analyzed from the perspective of energy,and the energy coordination relationship of this process is explored from the perspective of transfer,conversion and dissipation.The results show that the energy dissipation during the FPLRE start-up process is affected by the starting thrust,frictional load and sealing performance.The results show that the increase of starting thrust hinders the energy dissipation,while the friction load and piston sealing performance have a positive impact on it,that is,the energy dissipation in the starting process of FPLRE is weakened under the state of small friction load,good sealing performance and large starting thrust.(3)Focusing on the influence relationship between the starting condition and the first cycle,the first combustion cycle of FPLRE was explored by taking the operating performance,gas state,combustion performance and emission performance of the first cycle as the research origin.The results show that based on the starting condition of large starting thrust,the free piston does rapid movement in the same direction as the speed direction,compresses the fresh charge in the cylinder,the temperature and pressure in the cylinder rise,the temperature and pressure increase rate is large,it is easier to achieve the fire condition,and the temperature and pressure rise sharply after combustion.The gas flow intensity in the cylinder is high,and after reaching the fixed injection time,the fresh charge and fuel can be fully mixed to obtain a more uniform gas mixture.However,due to the rapid movement speed of the piston under the large starting condition,the combustion time is greatly shortened,incomplete combustion is more likely to occur,and the combustion integrity is low,which makes its performance in combustion performance and emission performance poor.