Research On Internal Factors Of Perfmance Degradation Of Two-Stroke Engine For Unmanned Aerial Vehicle

In the field of aero-engine,two-stroke spark ignition engine has the advantages of small volume,high power to weight ratio,simple structure and low price,which has become the main power device of small and medium-sized long endurance UAV.However,due to the harsh operating conditions,the performance of the two-stroke piston engine for UAV decays in use,and its decaying speed has a significant impact on the empty capacity of UAV.Therefore,it is of great significance to analyze the reasons for the performance degradation of the two-stroke engine for UAV to improve the engine design,compensate the attenuation on line and improve the performance of the engine.The 56 cc two-stroke two cylinder gasoline engine designed and manufactured by ourselves is taken as the research object in this study.The engine performance changes are observed through bench test.The short circuit phenomenon in the process of engine ventilation is tested by tracer gas method.The engine after 50 hours of operation is disassembled and checked.The phenomena of engine wear and carbon deposition are found.The influence of these phenomena on the performance degradation is analyzed by means of three-dimensional fluid dynamics simulation,and the evaluation of the effect is given.The results show that the maximum power and torque of the engine decrease by 69.9% and 51.3% respectively after 50 hours of operation.After the performance degradation,the air exchange performance of the engine deteriorates obviously,the trapping efficiency is between 31% ～ 55%,the scavenging efficiency is between 29% ～ 43%,and the charging efficiency is between 33% ～ 47%.Aiming at the phenomena of wear and carbon deposition found in the experiment,the contribution of the above phenomena to the performance degradation was studied by three-dimensional simulation.A three-dimensional numerical model for the fluid calculation of the crankcase scavenging cylinder exhaust system of a two-stroke gasoline engine was established.The parameters of the model were modified according to the actual wear and carbon deposition.The changes of the key processes such as the air exchange parameters and the gas flow in the cylinder before and after the performance degradation were compared and analyzed,and the specific effects of various factors on the air exchange performance were explored.The simulation results show that the scavenging efficiency has little change before and after engine performance degradation.The results show that the effects of piston leakage relative to exhaust port wear and piston surface carbon on the charging efficiency,fresh charge and trapping efficiency are greater,and the influence factors are as high as 0.727 ～ 0.924.