| The injectors and spark plugs of free piston engine(FPE)are mounted in the cylinder wall.Furthermore,the two-stroke cycle and uniflow scavenging result in shorter mixture formation and harder in-cylinder flow organization.In this paper,an opposed piston FPE is adopted,the scheme design and simulation analysis of in-cylinder flow and combustion process organization were carried out using CFD method.The research mainly covers the following aspects.(1)After analysing the special features of combustion chamber,single and dual channel in-cylinder flow organization schemes are proposed and the flow field distribution is analysed theoretically.Based on the spray-directed mixture organisation,the fuel injector and spark plug arrangement are designed.(2)A mesh model of the two-channel central combustion chamber is developed,spray and combustion models are determined and validate.Based on the CFD model,mesh and step size independent validation is carried out.Compared to the structural and unstructured meshes,the hybrid mesh capture more small vortices.When the mesh resolution reaches 12,the GCI valve is less than 6%and the smaller step size improves the simulation accuracy.(3)Mixture formation,combustion processes and pollutant emissions are compared for the single and dual channel combustion organization.The in-cylinder flow and residual exhaust gas distribution are analysed,and the fuel evaporation,the SMD and equivalent ratio distributions are compared.Different injection rates and spray cone angles are designed to analyse the stability of the single and dual channel combustion organization.(4)Based on the single-channel combustion organization,the effects of nominal compression ratio,initial fuel/air equivalent ratio and ignition advance angle on the hydrogen combustion are analysed.Finally,the effect of ignition advance angle on the gasoline combustion is analysed and the performance is compared.The study results show that the single-channel scheme has a high combustion efficiency of 98.4%,an indicated thermal efficiency of 39.5%,a circulating thermal efficiency of 40.2%,and a significant reduction in pollutant emissions.When the nominal compression ratio is 20,the initial fuel-to-air equivalent ratio is 0.95 and the ignition advance angle is 5~oCA BIDC,the hydrogen engine has the best comprehensive performance. |
