Study On Optimization Of Loop Scavenging And Combustion Characteristics Of Two-stroke Aero-engine

Two-stroke internal combustion engine has gradually become the preferred power for small and medium-sized UAVs due to its structural advantages and high power-toweight ratio.However,due to the lack of independent intake and exhaust process,the two-stroke engine has inherent problems of gas exchange and scavenging,so that the matching of engine fuel injection control and in-cylinder flow field has become a hot issue in the research of two-stroke engine combustion.In this thesis,a two-stroke loop scavenging engine is taken as the research object.Based on the co-simulation platform of GT-Power and ANSYS/Fluent,the factors affecting the gas exchange performance of the two-stroke loop scavenging engine are studied.Combined with the analysis of loop scavenging flow field,the influence of the change of fuel injection characteristic parameters on the formation of in-cylinder mixture was explored.The influence of ignition parameters on combustion characteristics of engine was explored.The main work contents and conclusions are as follows:1.The one-dimensional engine model was built based on GT-Power,and the boundary conditions required by the engine work were calculated and output.The cosimulation with Fluent was carried out,and the three-dimensional computational fluid dynamics(CFD)model based on the multi-process cylinder of gas exchange,fuel injection and combustion was established,and the verification was completed through bench test.2.The influence of scavenge ports axial inclination on scavenging efficiency of two-stroke engine is explored.The results show that the effective flow area of the scavenging ports can be increased by increasing the axial inclination appropriately,increase the fresh charge in the cylinder,and improve the scavenging efficiency of the engine.When the axial inclination of the scavenge ports is 32°,the scavenging efficiency can reach 98.8% and the charge efficiency is 95.5%.At the same time,increasing the axial inclination of the scavenge ports can improve the intensity of the tumble flow,increase the turbulent kinetic energy,and help to improve the evaporation rate of the fuel,so as to form a more uniform mixture in the cylinder.3.The influence of injection characteristic parameters on the formation and distribution of in-cylinder mixture is studied.The results show that reasonable increase of injection pressure can effectively increase the penetration distance of the oil beam,accelerate the fuel evaporation and improve the uniformity of the mixture in the cylinder.But too high injection pressure will cause the fuel to hit the cylinder wall,and the mixture uniformity decreases near top dead center.In addition,advance injection can increase the mixing time of fuel and air,improve the evaporation rate of fuel,improve the uniformity of the mixture in the cylinder,but premature injection will appear short-circuit loss of fuel phenomenon,increase the direct loss of fuel.4.The relationship between ignition parameters and combustion characteristics of two-stroke aeroengine is explored.It is found that effectively increasing the ignition energy can increase the size of the initial fire core,accelerate the flame propagation rate,increase the combustion explosion pressure,and thus improve the engine power.The flame travel distance is shortened and the engine can effectively reduce the risk of knocking.Delaying ignition timing makes the combustion starting point move back,resulting in more fuel combustion work in the expansion process,the expansion loss increases,and the maximum explosion pressure in the cylinder decreases,reducing the engine power.