Study On Dimension Limit Of Micro Free Piston Engine Based On Work Capacity Evaluation

At present,Micro-Elector-Mechanical System(MEMS)using hydrocarbon fuel leads the new era with its longer working time,higher energy density,smaller volume,easier to carry and no harm than other traditional batteries.Based on hydrocarbon fuel of characteristic length millimeter,MEMS can output power in the range of 1~100W MEMS has widespread applications whether in the field of life or in medical treatment.Among various micro power systems,this paper selected the Micro Free Piston Engine(Micro-FPE)with based on homogenous Charge Compression ignition(HCCI).The HCCI used in this engine can effectively solve the problem of flame quenching in the micro combustion chamber,which is the core component of MEMS and the easy miniaturization of the free piston engine can overcome the problem that the combustion process is difficult to control.With the miniaturization of Micro-FPE,the initial kinetic energy required to achieve ignition is also reduced.Therefore,the research of Micro-FPE operational size for HCCI has great significance to the development of MEMS.In this paper,the visual experiment bench of Micro-FPE is built.The combustion process of DME/oxygen in the micro combustion chamber was captured by a high speed digital camera and the motion of the free piston was analyzed by means of data processing system.The combustion characteristics of Micro-FPE under different compression ratios are analyzed.Experiments shows four combustion processes and its work capacity is evaluated by P-V diagram.In addition,the experiments of adding catalyst at the bottom of the micro-combustion chamber and preheating homogenous mixture show that the catalytic action can reduce the ignition point of homogeneous mixture and reduce the compression ratio of compressible combustion.The activation energy required for the compression ignition process in a working cycle is also reduced.The increase of the initial temperature of the mixture makes it easier for the mixture fuel to be compressed,and the working cycle time is shortened.It is concluded that both measures can reduce the compression compression ratio of Micro-FPE and broaden its operational dimension limit.In this paper,a numerical model of Micro-FPE based on HCCI with leakage gap is established.Methane and oxygen with high calorific value are used as fuel.The operational size limits of 1~10W/10~60W Micro-FPE were studied by the method of no catalysis no preheating,catalytic action,homogeneous mixture preheating and catalytic action.The work performance of Micro-FPE were evaluated by indicating work,net power and indicated thermal efficiency.It is showed that preheating and catalytic action were based on sacrificing the ability to do work,which broadened the limits of operational dimensions of Micro-FPE,but decreased its work ability.The minimum operating size of 1~10W Micro-FPE was as follows:the ratio of length to diameter was 40,the compression ratio was 38,the initial kinetic energy required for compression ignition was reduced by 38,the indicating work was 0.023J and the energy density was 271 MW/m~3.Compared with the traditional engine,the working volume utilization ratio of micro-FPE has been improved.And the energy density was obviously superior to that of other micro-power systems.The minimum operating size of 10~60W Micro-FPE is as follows:the ratio of length to diameter ratio was 16,the minimum operational compression ratio was 60,and the indicating work was 0.108 J,which was higher than that of 1~10W Micro-FPE.By analyzing the influence of leakage gap on the combustion characteristics and work capacity of Micro-FPE,the optimal leakage gap size and the selected critical gap size are obtained,which provided a theoretical basis for the size design of Micro-FPE.