Numerical Simulation On Influence Factors Of Combustion And Knock For Spark-Ignition Aviation Heavy Oil Piston Engine

Lightweight aviation piston engine plays an important role in general aviation and even military aviation because of the advantages of small size,high power per liter and simple structure.The early small aviation piston engine mostly used gasoline as fuel,but the safety risks of gasoline in storage and transportation are great because of its flammability,while aviation heavy oil(referred to as light diesel or aviation kerosene)has lower volatility,higher flash point,and better safety,which also meets the integration requirements for military fuels.Heavy oil piston engine is dominated by compression ignition,but it requires high compression ratio,resulting in its large size,weight and low power ratio.It reduces the practicality and flexibility for the small aviation field,so spark ignition will become the future development route for heavy oil piston engine.Due to the large difference in physicochemical characteristics of fuels,the ignition combustion characteristics of heavy oil and gasoline are different,and the higher knocking tendency of heavy oil greatly limits the power of the engine.This paper builds a CFD three-dimensional simulation model based on the experimental data of a four-stroke GDI gasoline engine,and uses large eddy simulation(LES)to carry out the simulation study of an spark-ignition aviation heavy oil piston engine.The control parameters of the combustion process enable the knocking control of aviation heavy oil in a spark-ignition piston engine.The main research contents and results are as follows:(1)A constant volume bomb CFD three-dimensional simulation model was established to study the spray pattern and penetration of a six-hole gasoline direct fuel injector in a constant volume bomb.The effectiveness of the spray model was verified based on experimental data.(2)The CFD three-dimensional simulation model of the engine's intake and combustion system was established.The model was calibrated by using experimental data.The ignition combustion characteristics and knock of diesel and gasoline were studied.The results showed that the diesel combustion reaction rate is slower than that of gasoline,and the knocking tendency is significantly greater than gasoline.(3)The effects of different control parameters on the combustion characteristics and knock of heavy oil spark-ignition piston engine were studied.The results showed: under the premise of ensuring the power performance,delaying ignition timing has a limited effect on knocking suppression.Within the concentration limit,the lean mixture's effect of suppressing knock is better than that of rich gas mixture,but it has greater impact on the power performance.Delaying injection time can suppress knock,but it will degrade the power performance.Proper secondary injection can effectively suppress knocking tendency and also obtain good power performance.(4)The influence of each control parameter on the spark-ignition combustion characteristics and power performance of heavy oil is as follows from more to less: injection time,ignition time,equivalence ratio,secondary injection;the impact on the knocking tendency is as follows from more to less: injection time,secondary injection,ignition time,equivalence ratio.Using the control strategies of ignition delay or equivalence ratio alone cannot simultaneously take into account normal combustion and power performance;proper injection timing can reduce the tendency of knocking and ensure good power performance at the same time;proper secondary injection strategies can be used to achieve normal state of combustion and power output.