Study On The Scavenging Process Of The Free-piston Engine Generator

Free-piston engine generator (FPEG), as a new powerplant, converts chemical energyinto electric energy by an efficient way. It has attracted considerable research interest for itslow mechanical loss, compact structure and short energy transfer path which means energyconservation and environmental protection. This paper focused on the research aboutscavenging process of FPEG. The research methodology was bench tests and systemsimulation.The results of experiments showed that: the combustion process of FPEG variatedcycle to cycle at low speed operation condition for the instability of scavenging process,however in traditional mode the engine that we used performed very well. In order toanalyze the differences between FPEG and traditional two-stroke engine on scavengingprocess, CFD model designed according to prototype was built. Calculation results showthat: the relative low scavenging box volume variation rate of FPEG is the effective factoron the scavenging efficiency, which leads to low-speed instability.In order to optimize the scavenging system, parameters including diameter of pistonassembly, volume of scavenging box, charging pressure, specific time-area values forscavenge and exhaust were investigated. Results show that: the low proportion of pistonassembly diameter on bore and high inlet pressure effectively improve the scavengingefficiency while the volume of scavenging box has slight influence on scavenging process;When we were designing the gas port, the value for scavenge should be modified with thetarget to the upper bound while the value for exhaust to the lower bound in the given rangeof specific time-area values.Finally, effects of operation conditions changing on scavenging process were studied.Research results show that: the change of operation conditions has no significant effects onthe scavenging process. With the operation frequency increasing, scavenging efficiencydecreases slightly and fuel-capture efficiency increases slightly. While, with thecompression ratio of FPEG increases, scavenging efficiency increased slightly andfuel-capture efficiency decreased slightly.