Simulation Research Of Gas Flow And Scavenging Quality For A Two-stroke Charged Engine

Two-stroke engine has been widely used on the small aviation aircraft such as unmanned aerial vehicle(UAV)owing to the advantages of simple structure,small size,light weight,high output per liter and low cost,etc.But with the increase of the altitude,atmospheric pressure,air density,the content of oxygen in per unit volume air decrease,causing the loss of power and torque and the increasing of the fuel consumption.The engine’s performance is rapidly deteriorated with the increase of altitude.Experiments showed that the power will drop about 8-10%,and the external characteristic fuel consumption will increase about 3-4% for the naturally aspirated engine with the altitude rising per 1000 meters.It is hard to meet the requirements that unmanned aerial vehicle could fly in the high altitude more than 6000 meters above sea level for long time.Engine with turbocharger could improve the power output and make the UAV flying at a high altitude.Traditional carburetor type or inlet jet crankcase scavenging two-stroke gasoline engine exists scavenging loss and after exhaust loss.Using turbocharging technology improves the scavenging pressure,increases the mix of old and new gas,and makes more fuel escape from the exhaust port,causing deterioration of fuel economy.Two-stroke gasoline engine with direct injection(GDI)technique could control fuel injection after exhaust port closed,largely reducing the short circuit loss of fuel.So turbocharger technology should be used in two-stroke GDI engine.This paper carried on the simulation research of turbocharger matching based on a two-stroke GDI engine,to reach the target of the power output at the altitude of 3000 m maintaining the same with the original machine on the ground and the power output at the altitude of 7000 m greater than 30% of the original machine on the ground.And making CFD calculation of the scavenging process for the specific point,analyzing the flow in the cylinder and judging the scavenging quality.The main research work of this paper include:(1)The establishment of the one-dimensional simulation model of the original two-stroke engine.The early stage of the experiment verified that the direct injection transformation has only little influence on the scavenging process of engine,so the pressure boundary condition of the direct injection engine model could be substituted by the condition of the original engine model considering that the direct inject engine model couldn’t built without the suitable injection module.The second chapter introduces the model building process.After correcting the model with the test data,the model can correctly reflect the working condition of engine;(2)Turbocharger was selected according to the boost target and the charged model was built without considering the effects of the exhaust energy.Simulate the charging model and obtain the boundary conditions which satisfies the requirement of the power output;(3)The CFD simulation analysis of straight engine scavenging process was carried using the boundary conditions obtained from the one-dimensional simulation.Comparing the flow before boosting with the flow after boosting at the altitude of 3000 m and 7000 m,and comparing the flow after boosting at the altitude of 3000 m with the flow before boosting on the ground.Analyze the scavenging quality and judge whether it meets the boost target;(4)The tuned exhaust pipe was redesigned and the CFD analysis was carried again.It proves that the engine performance could be better after the optimization of the tuned exhaust pipe.