Research On Performance Characteristics And Design Technology Of Free-piston Diesel Engine For Linear Generator

The free-piston linear generator (FPLG) which is a free-piston engine (FPE) workingin conjunction with a linear generator offers an interesting choice for use in hybrid vehicles.There are many performance advantages in it, such as simple structure, high conversionefficiency and power density, etc. But because it has a special motion of free-piston, whichcauses many other problems, it is still at the exploratory research stage. This paper, withcompression ignited free piston diesel linear generator (FPDG) as the research object, aimsto solve the key problems of FPLG in current study. The research on the design methods,performance characteristics, starting difficulty problem and operation mechanism of FPLGwas carried out. The main research contents and conclusions are as follows.1. Matching design method of structure parameters in FPDGThe research on free-piston dynamics was carried out as the main line in the designprocess. The matching relationship between the structure parameters and performanceparameters were studied based on the motion characteristics of free-piston and therelationship of energy transfer. According the matching relationship, a design method ofsystem parameters for FPDG was introduced. By using this method, the main structureparameters of a test prototype were determined. The effectiveness of the design method wasalso verified by simulation and test of the prototype. Moreover, according to thecharacteristics of free piston engine without crankshaft, a common rail fuel injection systemwas designed based on the feedback control of piston displacement for FPDG. The studyresults verify the validity of the design method; and also found that the operation frequencyof the free-piston is related to the stroke or the compression ratio of free-piston engine.2. Analysis of operating mechanism and performance characteristicsWith movement characteristics of piston as the starting point, using the method ofcoupling modeling, a detailed simulation of the scavenging process, combustion process,piston ring sealing and lubricating performances of free piston engine was presented, andcompared with a corresponding conventional crankshaft-driven engine to study thedifferent characteristics in these performances. The result indictaed that the scavenging efficiency and indicated efficiency of FPDG is lower than crankshaft engine, but theleakage loss and friction loss is less than that of the crankshaft engine.3. Start the process operation mechanism and performance characteristicsIn order to solve the start problem of compression ignited free piston diesel engine,using the resonance principle of mechanical system, the cumulative energy of compressionprocess was realized, the starting and combustion of the prototype was finished. Theperformances of starting process were obtained by experimental research. Moreover, thetheoretical analysis for the nonlinear dynamics characteristics of the free-piston during thestart-up process was carried out. The study results shows that the starting process of freepiston engine is self excitation vibration process, piston motion is gradually stable in thestart process; the changes of fuel injection advance displacement in free-piston engine haslittle effect on the piston motion, but injected fuel mass has an important influence on themotion of the piston in expansion stroke; with the increases of linear motor thrust, themaximum combustion pressure of the engine in starting process will be greater.4. Experimental research on free-piston diesel linear generator prototypeExperimental study on the work performances of prototype in different operation stage,including the first combustion cycle, the motored process conditions, focusing on thecombustion performances and piston movement status of free piston diesel engine lineargenerator work in different stages. According to the experimental data of the motoredprocess conditions, a detailed analysis of the combustion heat release of free piston dieselengine was presented. The research results indicated that motored condition, in-cylinderpressure and the compression ratio of engine is gradually increased, and then tended to bestable. For small bore free piston diesel engine in the motored process conditions, the maincombustion stage of combustion process is the rapid burning period, rather than slowburning period, and most of the fuel combustion with the premixed mode, rather thandiffusion combustion mode.