An Investigation Of Fuel Spray And Mixing Characteristics Of A Free Piston Diesel Engine Generator

The free piston engine generator(FPEG)mainly consists of two opposed free piston engines and a linear electrical machine.Compared with the conventional engine,the FPEG has been regarded as a very promising new power conversion device due to its special configuration and potential performance,such as structural simplicity,large power density,variable compression ratio,and low emission pollution as well as wide fuel adaptability.Since the FPEG cancels the crankshaft system,the piston movement law is variable and the motion speed is faster near the top dead center.For in-cylinder direct injection engines,the fuel atomization,evaporation and fuel-air mixing characteristics have an important effect on combustion and emission performance.The special piston motion characteristic of the FPEG may cause it to exhibit different spray and mixture formation characteristics than conventional diesel engines(CDE).Therefore,exploring the spray and fuel-air mixing characteristics of FPEG will help to further optimize its performance.This paper presents a combination of numerical simulation and experimental verification to study the FPEG spray and mixture formation characteristic.Under the conditions of injection quantity of 7mg,injection duration of 0.86 ms and equivalent speed of 1810r/min,the spray and mixing characteristics of FPEG and conventional diesel engine were compared and analyzed.And based on the spray and mixing characteristics of FPEG,the effects of different injection parameters and geometric parameters(injection rate shape,injection advance position,nozzle diameter and injection angle)on FPEG spray characteristics were studied to improve the in-cylinder mixture uniformity of FPEG.The main research contents and conclusions are as follows:(1)Firstly,a spray simulation model of FPEG which coupled with piston motion was established by using AVL_Fire software based on the original FPEG parameters and piston motion law,and compared with the experimental data obtained by spray visualization test,the correctness of the model was verified to be used for the research of spray process.(2)Compared with the conventional diesel engine,the injection timing of FPEG is later due to its special piston motion,and the atomization,evaporation,mixing and combustion processes of FPEG are relatively delayed while the injection position ofboth is the same.Due to the shorter spray penetration,the larger droplet sauter mean diameter and the weaker in-cylinder mean turbulent kinetic energy,the FPEG shows a slower fuel evaporation rate and a longer evaporation period,resulting in relatively less evaporated fuel.Besides,the air utilization rate of FPEG is always lower than that of the conventional diesel engine,indicating that the atomization effect is not well,and the mixture uniformity is poor;meanwhile,the FPEG shows lower peak of the in-cylinder gas cylinder pressure and temperature during the combustion process,the slower combustion rate as well as longer post-combustion period.(3)Under the same initial conditions,the effects of three injection rate shapes(left triangle,rectangular,right triangle)on spray and mixing characteristics of FPEG were compared and analyzed.Compared with the rectangle and right triangle injection mode,the left triangle injection rate shape shows the best level of atomization and mixing,meanwhile its combustion rate is faster after the start of combustion,and the peak values of pressure and temperature are the largest,indicating that the relatively fast injection rate at the early injection stage is more beneficial to improve the mixture formation in the cylinder,and enhance the level of isochoric heat release around top dead center.(4)Replace the injection timing with the fuel injection advance position,and set the five injection positions,from 5mm to 9mm,at intervals of 1mm to study the FPEG spray and mixing characteristic under the same initial conditions: when the fuel injection advance position is from 5mm to 8mm,the larger fuel injection advance position is conducive to the fuel atomization and evaporation.Besides,with the advance of injection process,the combustion rate is accelerated,and the peak values of pressure and temperature are continuously increased,and the times corresponding to the peak value are also advanced accordingly.When the fuel injection advance position is 9 mm,the air utilization rate is only slightly better than that when the fuel injection advance position is 8 mm.However,there are relatively more impinged fuel mass caused by the longer spray penetration when the fuel injection advance position is 9 mm,and the maximum of the heat release rate also drops.(5)Under the condition of same injection orifice area,properly reducing the nozzle diameter and increasing the number of nozzle hole are beneficial to the fuel atomization and evaporation process,further improving the mixing effect of fuel and air in the combustion chamber.When the nozzle diameter is 0.116 mm and the number of nozzle holes is 5,the effect of fuel atomization and mixing is better because of moreevaporated fuel mass during the ignition delay period.Besides,the heat release rate is faster,and the peak value of instantaneous heat release rate is larger,leading to the higher pressure and temperature.(6)The spray angle determines the development direction of the fuel spray and the drop points of the fuel in the combustion chamber.It is not conducive to the atomization,evaporation and mixing of the fuel if the spray angle is too large or too small.When the spray angle is from 130° to 170°,small spray angle will not only cause more impinged fuel mass,but also lead to lower fuel evaporation rate and air utilization rate due to the weaker in-cylinder airflow movement.However,when the spray angle is large,although the development of fuel spray is better,there is no obvious vortex formation in the cylinder,which is not conducive to the mixing of fuel and air in the cylinder,meanwhile a relatively large amount of rich mixture enters the combustion chamber clearance,resulting in the poor combustion effect.When the spray angle is 150°,not only the spray and mixing effect is better,but also the combustion condition is optimal.