| The rapid development of general aviation and UAV(Unmanned Aerial Vehicle)industry has an increasingly strong demand for small aviation power units.Because of its obvious advantages in fuel economy and cost,aviation piston engine plays a leading role in the field of medium,low speed and long-endurance UAV.Compared with aviation gasoline,aviation kerosene has the characteristics of high safety,simple logistics support,good fuel availability and low price.It also conforms to the military’s single fuel policy and has a broader application prospect.In recent years,piston engine fueled with aviation kerosene has become a key research direction in the field of military and aviation industry.Aiming at the problem that the poor evaporation of aviation kerosene in aviation piston engine leads to the decline of mixture quality and combustion efficiency,this paper establishes the geometric model of combustion chamber and CFD simulation model,and uses the professional simulation software AVL-FIRE to calculate the relevant parameters of fuel-airchamber matching in the combustion system of this aviation kerosene piston engine.The influence of injection position,spray cone angle,injection pressure,injection timing and fuel injection angle on the air flow,fuel evaporation,mixture distribution and combustion process in the 5500 r/min and full load cruising conditions were studied.Then,based on the calculated fuel injection parameters,the effects of ignition position and ignition timing on combustion performance and knock were simulated.Finally,the combustion system was improved based on the injection and ignition parameters with good matching between fuel-air-chamber.The main contents and conclusions are as follows:(1)The effects of fuel injection parameters on fuel air mixing and combustion characteristics of aviation kerosene DI engine were studied.Compared with the centrally mounted injector,the side-mountedd injector has better coupling between fuel spray and airflow.The evaporation,diffusion and mixing speed of fuel in the cylinder are faster,which can effectively improve the combustion quality of aviation kerosene,the ignition delay period and the combustion duration reduces by 1.5 °CA and 4 °CA respectively.When the spray cone angle is 45°,there is less fuel accumulation on the wall and fuel diffusion is the highest during spray impingement on the wall of the chamber,the reasonable distribution of flow velocity before and after ignition timing,and the high turbulence kinetic energy area mainly concentrates on the center of the cylinder,which is favorable for the propagation of the flame.For aviation kerosene piston engine,increasing the injection pressure can improve its atomization level and combustion process.When the injection pressure increases from 6 MPa to 16 MPa,the combustion burst pressure increases by 17% and the peak heat release rate increases by 12%.But when the injection pressure is greater than 16 MPa,the excessive spray penetration makes the fuel stick on the piston surface and the chamber wall,resulting in the increase of the concentration gradient of the mixture in the chamber and the deterioration of the combustion process.Under this cruise condition,injection at 70°CA,the fuel has enough time for evaporation and mixing in the cylinder.Therefore,the mixture in the cylinder is evenly distributed at the ignition timing,and the combustion heat release speed is fast.When the injection angle is 45 °,the spray fits well with the geometry of the chamber and the air flow in the cylinder.The mixture quality is the best,the combustion heat release speed is the fastest,the peak heat release rate increases by 10 J /(°CA),and the combustion burst pressure increases by 5.5%(2)The effects of ignition parameters on combustion and knock characteristics of aviation kerosene DI engine were studied.When the spark plug is centrally mounted,the flame propagation rate is the fastest and the ignition delay is the shortest,which is 23 ° CA.the combustion burst pressure increases by about 11%.The corresponding phase of the combustion burst pressure is 14 ° CA after TDC.As the flame propagation distance is shortened,the engine knock intensity is reduced.Under the cruising condition of the engine,delaying the ignition time can effectively reduce the knock intensity,but the combustion center of gravity moves backward,the aftercombustion increases,more fuel burns and does work in the expansion process,the expansion loss increases,the combustion equal volume decreases,and the maximum explosion pressure decreases.When the ignition timing is 340°CA,the combustion process organization is relatively timely,and thermal power conversion efficiency are higher.(3)The better results of fuel injection parameters and ignition parameters are selected to improve and compare the performance of the combustion system of aviation kerosene piston engine.The coupling characteristics of in-cylinder air flow and fuel injection parameters of the optimiazed engine are improved,the tumble intensity is increased,the mixing process of fuel and air is improved,and the formation quality of in cylinder mixture is improved,which is close to the equivalent mixture concentration.Moreover,the tumble motion is maintained well near the end of compression,the turbulent kinetic energy is improved,and the flame propagation speed is accelerated.Therefore,the combustion volume of the improved combustion system is increased,and advances the combustion phase.The combustion center of gravity was advanced by 2 °CA,the combustion duration was shortened by 3 ° CA,the peak instantaneous heat release rate increases from 55.56 J /(°CA)to 67.03 J/(°CA),the maximum combustion pressure in the cylinder increases from 5.53 MPa to 6.25 MPa,the maximum combustion pressure corresponding to the crank angle is advanced by 1.5 °CA,the maximum pressure rise rate is 0.33 MPA /(° CA),and the combustion quality is improved. |
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