| In the context of increasingly severe energy and environmental issues,a kind of energy conversion device called free-piston electromechanical hybrid system(FEHS),with its unique structure and performance advantages,has been stand out from many devices and is favored by researchers in related fields.With the deepening of research,many studies have found that the difficulty of maintaining the stable operation of the FEHS prototype is an important reason for hindering the further development of the system.Some researchers have pointed out that one of the crux of the prototype is the mixture preparation instability and the combustion fluctuations,caused by the instability of gas exchange.At the same time,some research institutions began to pay attention to the research work of FEHS gas exchange.However,most of these studies focused on discussing the influence of ventilation structure and operating parameters on the stability of FEHS gas exchange,while ignoring the variation of piston motion on the stability of FEHS gas exchange.It should be noted that the most significant feature that distinguishes FEHS from crankshaft engines is its relatively free piston motion,and the current FEHS gas exchange structure is mainly a scavenging port type.Therefore,the variable piston movement is not only the most significant feature of FEHS,but also the direct cause that affects the opening and closing of the system’s intake and exhaust ports,which is bound to have an important impact on the gas exchange stability of FEHS.In this context,in order to ascertain the influence of piston motion variation on FEHS gas exchang performance,and to provide a theoretical basis for FEHS gas exchange stability control based on piston motion control,this paper combines the piston motion characteristics of FHES and proposes the study about the effect of variable piston motion on the gas exchange stability of FEHS.The MATLAB/Simulink and AVL＿FIRE software platforms were utilized to carry out joint simulation,the FEHS machine model was extablished to obtain the piston motion;the FEHS three-dimensional computational fluid dynamics model was built to accurately simulate the FEHS spray,combustion and gas exchange process;the accuracy of the whole machine model and the gas circulation of the scavenging structure were verified;the effect of the variable piston motion,including variable stroke and variable trajectory on FEHS gas exchange stability was investigated;the response sensitivity of the gas exchange parameters caused by the piston movement changes was analyzed.The conclusions obtained are as follows:(1)Increasing the piston stroke is not conducive to improving the combustion performance of FEHS,resulting in the reduction of piston movement speed,but providing sufficient gas exchange duration for FEHS,promoting the removal of residual exhaust gas and increasing the in-cylinder fresh charge,and improving the scavenging efficiency,but also reduces the trapping efficiency.Among them,when the stroke is 82 mm,the scavenging efficiency is 92.9%,and the trapping efficiency is 56.0%.When the stroke is increased to 92 mm,the scavenging efficiency reaches 98.1%,and the capture efficiency drops to 30.6%.(2)Increasing the piston trajectory shape parameter Ω is conducive to increasing the residence time of the piston near TDC and BDC,promoting isometric combustion,and improving combustion pressure and temperature.But higher temperature and pressure also lead to higher heat loss,leading to a decrease in the initial exhaust pressure and temperature.In addition,the longer residence time near the piston BDC provides FEHS with longer opening duration of the exhaust and intake ports,which creates favorable conditions for more fresh charge to enter the cylinder,which is conducive to sweeping more residual exhaust gas out of the cylinder,but inevitably resulting in a reduction in trapping efficiency.When the parameter Ω is 0.4,the capture efficiency is 55.75%,and when the parameter Ω is increased to 1.6,the capture efficiency is reduced to 27.60%.(3)Sensitivity analysis results indicate that the sensitivity coefficients of peak pressure,peak temperature,combustion efficiency,exhaust pressure,and exhaust temperature increase with the increase of piston stroke,but decrease with the increase of shape parameter Ω.The trapping efficiency and residual exhaust gas are sensitive to changes in the variable motion,but the scavenge efficiency,trapping efficiency,and fresh charge increase with the increase of the piston stroke and shape parameter Ω,and the sensitivity coefficient gradually decreases. |
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