Experimental And Simulation Study On Supercharged Combustion Process Of Spark Ignition Free Piston Engine

Supercharging is a common method of increasing engine power density.The spark ignition hydraulic free piston engine eliminates the crank mechanism and is structurally more adaptable to high power density.However,due to the uniqueness of the free piston motion law,the combustion process and piston motion law of the supercharged hydraulic free piston engine will change greatly.In this thesis,the characteristics of the piston motion law,combustion process and knock tendency of a supercharged spark ignition hydraulic free piston engine are studied by experiments and simulations to explore the optimization methods of high efficiency and high power.In the experiment,a test bench for the spark ignition hydraulic free piston engine was built,which can simulate the supercharged combustion process.In the simulation,the model of the spark ignition hydraulic free piston engine was established by a 3D simulation software.The coupling of the piston motion and the combustion process is achieved in the simulation.The experimental results show that the piston movement cycle is shortened when the hydraulic free piston engine is supercharged,and the residence time near the top dead center is also shortened accordingly.Appropriately increasing the initial pressure in the cylinder is beneficial to improve the indicating efficiency.In the experiment,as the initial pressure in the cylinder was increased to 0.15 MPa,the highest indication efficiency was increased from 30.0% to 31.5%.With further increasing the initial pressure in the cylinder to 0.20 MPa,the highest indication efficiency dropped back to 30.5%.According to the experiment and simulation results,the supercharging of the spark ignition hydraulic free piston engine is beneficial to suppress knocking combustion.Although the energy density in the cylinder is increased when supercharged,the piston motion cycle is shortened at the same time.Under the combined effect of the two factors,the increase of the initial pressure in the cylinder shows the effect of suppressing knock.In terms of knock intensity,the effect of high energy density plays a major role,so that the knock intensity is improved when supercharged.It is verified by experiments that the self-adaptation of the compression ratio of hydraulic free piston engine with the ignition phase has the effect of suppressing the process from pre-ignition to super-knock.With the initial in-cylinder pressure increased,the self-adaptation is reduced,but the piston motion cycle is also shortened accordingly.In the simulation,the mass of the piston group is doubled when the initial pressure in the cylinder is 0.2 MPa,and the results show that the adaptability is improved.Aiming at the low indication efficiency under the condition of high initial pressure in the cylinder of hydraulic free piston engine,two schemes of optimizing piston mass and multi-spark ignition are proposed.The simulation results show that the indication efficiency can be increased from 31.3% to 39.9% by optimizing the weight of the piston set under the condition of initial pressure of 0.2 MPa in the cylinder.Under the conditions of initial pressure of 0.4MPa or 0.8MPa in the cylinder,the four-spark ignition can effectively improve the indication efficiency.The simulation results show that multi-spark ignition can effectively improve the indication efficiency without knocking under high supercharged condition.