Numerical Studies On The Influences Of EGR And Miller Cycle On The Knock Characteristics Of Piston Aeroengine

As the power source of UAV,aviation kerosene piston engine has obvious advantages in safety,versatility and fuel availability compared with traditional gasoline aviation piston engine,so aviation kerosene as the fuel of aviation engine is widely concerned by researchers.However,the auto ignition temperature and octane number of kerosene is low and it is difficult to atomize.Therefore,the kerosene is often faced with the problems of combustion stability at small load and knock combustion under heavy load in practical application.Therefore,one of the research hotspots of aviation kerosene piston engine is the knock suppression under the condition of large load,which will be studied in this paper.At present,there are many researches on the knock suppression strategy of gasoline engine,but few on the knock suppression of aviation kerosene engine.It is of great practical significance and engineering application value for the development of aviation kerosene engine to carry out the research in this field and to widen the knock limit by adopting knock suppression measures so as to improve the engine power.In this paper,a three-dimensional simulation model of piston aero-engine is built in the software of converse.After the calibration of the model,the knock combustion process of low octane fuel is simulated and analyzed.On this basis,the inhibition effect of different EGR rate on detonation was studied.It is found that the effect of EGR technology on knock suppression of low octane fuel is obvious.After the introduction of EGR gas,due to the dilution effect and thermal effect,the combustion reaction rate of the combustible mixture slows down,the flame propagation speed decreases,and the in-cylinder temperature decreases,so as to effectively inhibit the spontaneous combustion in the combustion chamber.When the EGR rate increases to 12%,the amplitude of pressure oscillation in cylinder decreases by 99.2% compared with that when the EGR rate is 0.After that,the dilution effect and thermal effect of EGR gas are studied deeply.It is found that the effect of EGR gas on reducing the temperature in cylinder and suppressing knock mainly comes from its dilution effect,while the thermal effect is less.Miller cycle can reduce the effective compression ratio of the engine.In this study,the effective compression ratio of the engine decreases from 4.2 to 2.2 as the LIVC increases from 10 CAD to 40 CAD.The decrease of the effective compression ratio will reduce the in-cylinder temperature during the compression stroke,which will slow down the reaction rate after the combustion starts,reduce the temperature during the combustion process,inhibit the spontaneous combustion of the end mixture,and reduce the knock intensity.When the intake valve closure time was delayed 20 CAD,the pressure oscillation in the cylinder decreased 96.3%.Delaying the closing time of the intake valve is helpful to enhance the intake stroke and the rolling flow in the cylinder at the beginning of the compression stroke,so that the turbulent kinetic energy in the cylinder before ignition is slightly increased,which is helpful to speed up the oil-gas mixing rate and improve the ignition stability.After increasing the intake pressure on the basis of Miller Cycle,the quality of fresh mixture entering the combustion chamber increased significantly,and the IMEP of the engine returned to the level before Miller cycle technology.The in-cylinder temperature decreases with the increase of intake air pressure,so the Miller cycle can improve the engine power performance on the basis of achieving better knock suppression effect.In a word,EGR technology and Miller cycle technology have significant knock suppression effect on low octane fuel.This conclusion has guiding significance for the follow-up development of aviation kerosene engine.