| With the rapid development of the navigation industry and the field of unmanned aerial vehicles,the demand for power devices is increasing.As the earliest developed power unit in the field of general aviation,aviation piston engine has the advantages of simple structure,mature technology and easy maintenance,and plays a major role in the field of low-altitude and low-speed small aviation.Compared with aviation gasoline,aviation kerosene has the advantages of high flash point,high density,good safety,and easy storage.It has attracted more and more attention in military and aviation fields,and has gradually become the main fuel for small and medium-sized aviation piston engines.The wide application of aviation kerosene is conducive to promoting the integration of military fuel.In view of the difficulties of evaporation and atomization,uneven mixture of oil and gas,and poor combustion effect of aviation piston engine fueled with kerosene.From the perspective of combustion chamber structure,a simulation model based on the combustion system of the prototype were established,and the influence of different combustion chamber types on the combustion performance of the engine were studied.Combined with the combustion chamber with good combustion performance,the wavy and stepped combustion chambers were designed.The effects of the depth of the central and side bosses on the combustion performance of the wavy combustion chamber were studied,and the effects of lip radius and lip depth on the combustion performance of the stepped combustion chamber were also studied.Finally,based on the analysis results of different combustion chamber types and structural parameters,a combustion chamber improvement scheme was proposed to improve the combustion organization of kerosene in the cylinder.The main work and conclusions are as follows:(1)The effects of four types of combustion chambers on the combustion performance of an in cylinder direct injection aviation kerosene piston engine were studied,include open type,straight type,shallow type andωtype.Compared with the original open combustion chamberωthe combustion performance of the combustion chamber is the best.While retaining the open characteristics,a central boss is added at the bottom of the combustion chamber along the tumble axis.This design effectively increases the turbulent kinetic energy in the cylinder,plays a good wall guide for the air flow in the cylinder,is conducive to promoting the process of oil-gas mixing and improving the distribution of mixture in the cylinder.Although the throat diameter of the shallow type combustion chamber is increased,due to the increase in the depth of the combustion chamber,the kinetic energy loss is faster when the air flow moves to the top wall of the piston and returns,the turbulence intensity in the cylinder is lower,the flame propagation speed is slow,the spontaneous combustion probability of the mixture at the end of the combustion chamber is increased,the knock tendency is intensified,and the combustion effect is poor.(2)The effects of the depth of the central and side bosses of the wavy combustion chamber on the formation and combustion process of the mixture in the cylinder of a direct injection aviation kerosene piston engine were studied.For the wavy combustion chamber,with the depth of the central boss increasing from 1.33 mm to 3.67 mm,the maximum explosion pressure in the cylinder first increased from 5.94 MPa to 6.88 MPa,and then decreased to 6.36 MPa,When the depth of the central boss is 2.65 mm,the maximum explosion pressure in the cylinder reaches a peak value of 6.88 MPa,and the maximum explosion pressure phase first advanced and then delayed.When the depth of the central boss is 2.65 mm,the fuel evaporation rate in the cylinder is fast.After the fuel evaporation,there is enough time to fully mix with the air,and a mixture of appropriate concentration is formed at the spark plug position,which is conducive to accelerating the formation of the core after the spark plug jumps,shortening the ignition delay period,reducing the possibility of spontaneous combustion of the mixture at the end,and reducing the knock intensity.When the depth of the central boss is constant,properly increasing the depth of the side boss can enhance the disturbance of the air flow in the cylinder,accelerate the formation of the tumble organization,and reduce the impact of the oil and gas beam fluctuation on the air flow movement in the cylinder.(3)The effects of the lip radius and lip depth of the stepped combustion chamber on the air flow,mixture formation and flame propagation in the cylinder of a direct injection aviation kerosene piston engine were studied.For the stepped combustion chamber,the maximum turbulent kinetic energy in the cylinder increases from 112.6 m~2/s~2 to 121.7 m~2/s~2 as the lip radius increases from 1.25 mm to 2.53 mm,and the peak turbulent kinetic energy in the cylinder decreases to 116.6 m~2/s~2 as the lip radius continues to increase to 3.95 mm.When the lip radius is 2.53 mm,the overall turbulent kinetic energy level in the cylinder is relatively high,and the high turbulent kinetic energy area is located in the center of the cylinder,which is conducive to accelerating the propagation speed of the flame front.The fuel mixture in the center of the combustion chamber releases a lot of heat rapidly,and the combustion organization is relatively sufficient,so the peak heat release rate is 50.2 J/(°CA).Under the condition of constant lip radius,the greater the lip depth,the lower the gas flow rate in the cylinder,the slower the diffusion speed after fuel evaporation,the more areas of rich and lean mixture in the cylinder,the longer the ignition delay,the higher the probability of spontaneous combustion of the mixture far from the spark plug,and the greater the knock tendency.As the lip depth increases from 1 mm to 5 mm,the maximum pressure in the cylinder decreases from 6.99 MPa to 6.47 MPa,the peak pressure rise rate decreases from 0.32 MPa/(°CA)to 0.27 MPa/(°CA),and the maximum cylinder pressure phase is delayed by 2°CA.(4)The structure and performance of the combustion chamber of the aviation kerosene piston engine were improved and compared by selecting the wavy and stepped combustion chambers with more uniform mixture distribution and better combustion effect in the cylinder.The improved wavy combustion chamber enhances the turbulent kinetic energy in the cylinder,has a good tumble organization,improves the formation quality of the mixture,and accelerates the formation of the core and the propagation rate of the flame surface.Compared with the prototype combustion chamber,the maximum in-cylinder explosion pressure of the stepped combustion chamber increased by 10%from 6.36 MPa to 6.99 MPa,And the occurrence time of the maximum cylinder pressure is advanced from 15°CA after top dead center to 13°CA after top dead center;The peak heat release rate of combustion increased to 52.26 J/(°CA),and the phase of the peak heat release rate was advanced by 1.5°CA.The improved wavy and stepped combustion chambers have concentrated combustion phases,increased isovolumetric capacity,and a maximum pressure rise rate of 0.32 MPa/(°CA).The knocking tendency in the cylinder is low,and the combustion organization is improved. |
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