Aerodynamic Design Of Turbocharging System For A Small Altitude Two Stroke Engine

With the increasing application of unmanned aerial vehicle(UAV)in the field of military,civil and scientific research,the performance demand of UAV engine is having been increasing.At present,twostroke engine,as a small and medium-sized UAV power plant,is widely used.Due to air is thin at high altitude,the engine don’t have enough intake gas.the effective power of the engine is reduced greatly,the fuel consumption rate increased significantly,it is difficult to meet the UAV task requirements.Turbocharging technology,as an important means of engine power recovery,can improve engine’s effective power at high altitude intake conditions,reduce fuel consumption and make the engine have good adaptability at high altitude.In this paper,a three cylinder two stroke aviation piston engine is selected as the research object,and the one-dimensional performance simulation model of the engine is established by using GT-power software,and its reliability is verified.According to the mission requirement of UAV 7000 m flight ceiling,the design operating point of engine and the target power after pressurization.Combining with the original engine one-dimensional performance simulation model,the turbocharger type of the engine is determined as the exhaust gas turbocharger.And then the design parameters of the centrifugal compressor and the centripetal turbine is calculated,which is used for the turbocharger design.According to the total design parameters,the thermodynamic calculation of centrifugal compressor and centripetal turbine is made by self programming,and the geometric parameters and speed parameters of its inlet and outlet are obtained.The geometric model of the centrifugal compressor and the centripetal turbine is made,and the three-dimensional geometric model of the compressor and the turbine is obtained.The numerical simulation of the geometric modeling is carried out by using the NUMECA software.The flow status of the design point of the compressor and the turbine is analyzed and compared with the original hypothesis.The performance of the non design points is calculated and used to draw the characteristics map of the compressor and the turbine.Using GT-power software,the one dimension performance simulation model of the engine was pressurized and transformed to match the engine with the turbocharger.It is found that the matching of turbocharger and engine is good.Most of the operating points of engine are located in the high efficiency area of compressor and turbine characteristic map,and far away from surge line.The effective power of turbocharged engine at high altitude intake condition is significantly higher than that of the original engine,and the fuel consumption rate is lower,which is more suitable for UAVs with high altitude long haul mission needs.Aiming at the characteristics of exhaust system of two stroke engine,the pressure wave pattern of cylinder exhaust port before and after pressurization is analyzed and studied.It is found that exhaust pipe and engine’s tuning speed is change between pressurized engine and original engine.On the exhaust pipe pressure wave propagation of unsteady numerical simulation method,confirmed the threedimensional unsteady dynamic row can capture the pipe pressure wave reflection characteristics are calculated.The simulation results show that in order to better realize the matching of the exhaust pipe and the turbocharged engine,it is necessary to optimize the design of the exhaust pipe.