Numerical Simulation Study On Scavenging And Combustion Process Of Two-stroke Stepped Piston Aeroengine

Unmanned aerial vehicle(UAV)currently generally use aero-gasoline engines as power devices,while naval vessels,aircraft,tanks and other equipment use heavy oil engines as power devices.The use of heavy oil for UAV can not only meet the requirements of national defense equipment for fuel Unity,but also has the advantages of high flash point,non-volatile and safer storage and transportation compared with gasoline.Because the two-stroke engine has higher lift power than the four-stroke engine with the same displacement,and has simple structure,light weight and high power-weight ratio,it is widely used in small UAVs.However,the traditional two-stroke aeroengine has some fatal shortcomings,such as low scavenging efficiency,poor altitude characteristics,low durability and high emission.For this reason,a new two-stroke stepped piston uniflow scavenging heavy oil engine is designed in order to obtain better performance.The scavenging process of a two-stroke engine has an important influence on the performance of the engine.Based on this,the uniflow scavenging process and diesel combustion process of the newly designed two-stroke stepped piston engine are numerically simulated.The effects of Swirl Orientation Angle(SOA)and stepped piston diameter ratio on scavenging performance were studied.Several typical injection times(mid-scavenging stage,end-stage,mid-compression stroke and end-stage)were selected to study the effects of different injection timing on equivalence ratio distribution in cylinder and combustion process.In addition,the effects of different ignition time and spark plug arrangement on combustion characteristics were also studied.Influence.The main conclusions are as follows:(1)The uniflow scavenging efficiency of two-stroke stepped piston engine is more than 95%.Appropriate SOA can reduce the mixing of fresh charge and exhaust gas and effectively improve the scavenging efficiency of two-stroke stepped piston engine.(2)The strength of swirl in cylinder increases with the increase of SOA.A larger SOA is beneficial to scavenging the early exhaust gas,but it deteriorates later.The residual exhaust gas in the middle of the cylinder is not easy to be scavenged,while a smaller SOA is prone to fresh charge short circuit in the scavenging process.The optimum SOA increases slightly with the increase of the diameter ratio of the stepped piston.(3)The scavenging efficiency and the pump power of the engine increase with the increase of the diameter ratio of the stepped piston.Therefore,choosing a reasonable diameter ratio of the stepped piston can improve the power and economy of the engine at the same time.When the diameter ratio of stepped piston is 1.75,the optimum SOA is about 11.2 degrees.(4)Fuel evaporation and mixing quality are better when injection timing is in the middle of compression stroke(SOI=280°CA).At the ignition time,the mixture concentration is moderate and the distribution is relatively uniform.It is suitable to spark ignition and flame propagation.The peak heat release rate and average pressure in cylinder are high.(5)With the ignition delay,the peak values of average pressure and heat release rate in cylinder decrease;when the double spark plugs are symmetrically arranged at X=+0.022 positions on both sides of Y axis,the flame propagates faster,the peak values of heat release rate and average pressure in cylinder are higher,which is the recommended spark plug layout scheme.