Investigation On The Starting And Operation Processes Of A Spark-ignited Free-piston Engine Generator

With the current energy and environmental pollution issues become more and more serious,as a novel hybrid power device,the Free Piston Engine Generator(FPEG)is attracting attention due to its advantages of compact design,multi-fuel feasibility,high power conversion efficiency,etc.In this research,a spark ignition FPEG with dual-cylinder dualpiston configuration is employed,both simulation and experimental study has been undertaken to investigate the engine cold start-up process,and combustion process.The main research objectives and conclusions are listed below:(1)A dual-cylinder dual piston spark ignition FPEG prototype and testing bench is developed,based on the system power matching design method.The design and optimisation methodology of the free-piston engine and the linear electric machine sub-systems is proposed,and the control strategy for the engine cold start-up process,stable working process,and the integrated system control strategy are proposed and discussed.(2)In order to solve the problem for the engine cold start-up of the dual-cylinder dualpiston spark ignition FPEG,a starting strategy with mechanical resonance is proposed.The piston reciprocates during the starting process,and the compression energy of the in-cylinder mixture will grow gradually.The linear machine is operated as a motor and outputs a constant force in the same direction of the piston velocity.Simulation results indicate that a closedloop control strategy is required for this starting control strategy,the actual motor output thrust is then able to main the target constant value.(3)The operation characteristics of the FPEG during the starting process with mechanical resonance is simulated,and the possibility of the proposed strategy is verified.The influence of different motor forces on the engine performance is investigated.By improving the thrust force of the linear motor,the changing rate of the peak cylinder pressure is increasing,and higher peak cylinder pressure can be achieved during stable state.The piston top dead centre or engine compression ratio is higher with higher motor force during the starting process,and less operation cycles are required to complete the free piston engine cold start-up process.(4)A numerical model of the FPEG during the stable operation process is developed and validated using test data from the prototype.Simulation results indicate that the piston peak velocity and engine compression ratio are proportional to the throttle opening area.The heat release process of the FPEG is close to a constant volume process.The piston velocity is high after its top dead centre,which will help reduce the heat transfer loss during the expansion process,and cut down the emissions relevant to high in-cylinder temperature.The system operation speed is low,and achieved FPEG engine efficiency is up to 35% with a power output of approximately of 4 kW,and the frictional loss is less than 5% of the indicated power.(5)Experimental investigated is undertaken on the developed FPEG prototype during the engine cold start-up process,combustion process.During the starting process,the peak in-cylinder pressure and engine equivalent speed are nearly proportional to the thrust force from the linear electric machine.In order to ignite the in-cylinder mixture successfully,the motor force should be higher than 103 N.During the combustion process,the engine incylinder pressure grows rapidly to 40 bar in the fourth operation cycle during the starting process,and the engine compression ratio achieved is higher than 9:1,indicating that the gas fuel mixture is ignited successfully.(6)The FPEG system dynamics model is simplified to a forced vibration equation,i.e.m(?)+ c(?)+ kx=F(t),and the solution for the piston displacement is obtained.The simplified model decoupled the design parameters for the FPEG operation performance.In this research,the influence of the piston area,the moving mass of the piston assembly and the mover,compression stroke length,and the coefficient of the electric load resistance on the system operation characteristics i.e.piston amplitude,peak velocity,peak acceleration,engine speed,maximum power output.This will provide a guidance to the FPEG system design process,and the following research on the different sizes of the FPEG.