| Hydrogen has the characteristics of high calorific value,wide ignition limit,fast combustion and clean combustion.Hydrogen fuel free piston hybrid system(HFPHS)has the advantages of high energy conversion efficiency,clean combustion,low pollution emission and broad prospects compared with traditional fossil fuel piston engine system.However,due to the strong coupling relationship between the physical field processes of HFPHS,the current models and methods cannot effectively describe the coupling performance of HFPHS,especially the coupling mechanism and coupling influence law between ignition position and thermodynamic performance have not been studied in detail.In this paper,spark ignition and two-stroke reflux scavenging HFPHS are taken as the research object.The coupling relationship between piston motion,combustion process,gas exchange process and electromagnetic conversion multi-physical field process was analyzed.The HFPHS machine-electric-flow-thermal coupling model was constructed,and the coupling mechanism between ignition position and thermodynamic performance was explored.At the same time,a HFPHS multi-dimensional cylinder fluid dynamic mesh model was established.The piston motion and thermodynamic process of HFPHS under the change of ignition position were calculated iteratively,and the piston motion under the change of ignition position was revealed.The specific research contents are as follows :1.The coupling relationship between piston motion and fuel mixing under the change of ignition positionUsing the established multi-field coupling model,the influence of ignition position change on the piston motion and hydrogen fuel mixing process of HFPHS was studied,and the variation law and coupling relationship of piston motion and fuel mixing process under ignition position change were explored.The results showed that the velocity and acceleration of the piston show a convex trend with the advance of the ignition position,and there is a peak at 3mm,which promotes the enhancement of the airflow velocity and turbulent kinetic energy in the mixing process.The hydrogen mixing stroke decreases linearly with the advance of the ignition position,resulting in a more uniform hydrogen fuel mixture at the later ignition position.2.The combustion and emission performance characteristics and coupling law under the change of ignition positionAccording to the change of piston motion boundary and fuel mixing boundary caused by the change of ignition position,the variation characteristics of combustion boundary parameters with ignition position were studied,and the variation law and coupling relationship of combustion and emission in cylinder under the change of ignition position were explored.The results showed that the in-cylinder pressure,pressure rise rate,heat release rate,indicated work and thermal efficiency have a convex relationship with the ignition position.The indicated work and thermal efficiency have peaks at 3mm,which are 117.2J and 37.7 %,respectively.The in-cylinder temperature and cumulative heat release are positively correlated with the ignition position,which leads to higher NO emissions at the earlier ignition position.3.The sensitivity analysis of heat transfer and combustion parameters under the change of ignition positionDue to the multi-field coupling characteristics of HFPHS,the change of thermodynamic state in cylinder has an important influence on its heat transfer process.Therefore,the influence of ignition position change on heat transfer process in cylinder is studied.In addition,the sensitive response of combustion parameters to ignition position change was also analyzed.The results showed that the heat transfer loss in cylinder increases linearly with the advance of ignition position due to the high temperature and heat release in cylinder.In the sensitivity response analysis,the fluctuation degree of the parameter change rate of the peak pressure is the largest,and the peak temperature,peak pressure and ignition advance angle have higher sensitivity. |
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