Research On Key Problems For Continuous Work Of Compression Ignition Free-piston Engine Based On Linear Generator

Free-piston engine linear generator(FPELG)highly integrates the engine and linear machine.It's a new solution and device of energy transformation.Under the background of saving energy and protecting environmental,numerous research institutes are constantly explore its potential advantages such as high efficiency,low emission and general applicability of fuel.At present,the research work is in interim period from exploratory to engineering test.Various scientific and technical problems have become bottlenecks which restrict its further development.This paper focuses on the compression ignition prototype of FPELG.It does research and solves many problems which influence system continuous operation.A simulation model which covers overall work conditions is developed.It considers and couples the F-v characteristics of linear motor.The mechanism of linear motor to start the system is ascertained clearly.A method which can transfer reference coordinate is proposed to analyze heat release and cyclical variation characteristics of FPELG.The nozzle and injection controlling parameters are optimization based on the fuel injection experiments and simulation.Two feedback control strategies of injection are applied on prototype and test rig.Finally the system can work continuously and its stability is improved significantly.The main research contents and conclusions are as follows.(1)The self-balance mechanism in starting process cuased by F-v characteristics of linear motor is ascertained.The relationship of structure parameters,electromagnetic characteristic parameters,F-v characteristics and operation of engine is established totally.The magnetic flux density B is determined by structure of linear motor,and it is positively related to velocity v of mover.The electric loading A is determined by coil current,and it is positively related to motor output force F.The mover velocity affects cylinder pressure directly after self-balance process.It is also a key parameter to guarantee starting of system.The force relates to the starting cycles.(2)Three methods which can transfer reference coordinate from time to crank-angle are proposed.The analysis of kinetic characteristics in crank-angle is improved.The heat release characteristics and differences with conventional engine in different stagies are analyzed by experiment.Comparing with conventional engine,it is discovered that the starting burning time is advanced,fuel consumption percentage of pre-mix combustion is higher.The rapidly drop in early expansion stroke of cylinder pressure is one of the major reasons which leads to low efficiency.The efficiency can be improved by enlarging stroke of FPELG.The constant volume combustion can be improved by increasing the injection advance angle.(3)The combustion cyclical variation of engine is analyzed by experiment.The cylinder pressure is higher in initial process after starting.Then it reduces rapidly until stable.The corresponding crank-angle of peak cylinder pressure distributes widely.It is more concentrate when the leakproofness of cylinder is better.The coefficient of variation(COV)of the two cylinders can reach up to 27.3% and 29.6%.The cyclical variation of peak pressure and peak pressure rising ratio is heavier than conventional engine.The cylinder pressure datas are reconstituted in phase space.The curve is projected on plane X-Y.It is found that the separation of path line caused by heat releasing is not clearly.The pressure rising ratio in expansion stroke is higher than conventional engine.(4)The injection characteristics of different nozzles are tested.The cyclical injection mass can be reduced by shrinking nozzle and reducing number of nozzles.Meanwhile the injection pulse width is also meet requirements of injection stable.The CFD mesh model of cylinder is modeled and modified based on experiment results.The influence of nozzle parameters and injection controlling parameters on heat release process is studied.Also the parameters are optimized to improve efficiency and stability of FPELG.(5)Two feedback control strategies of injection are proposed with position and velocity of piston as feedback variables respectively.From energy transferring perspective,the two strategies essentially control compressive energy in cylinder before of after injection.Furtherly,the influence of controlling parameters on engine performance is studied.It is finally found that the feedback control strategy based on piston velocity can not only keep continuous working but also improve the stability of FPELG.